On thing to keep in mind when using a domino is to always reference from the same face (or plane, in case that helps) or the mortice won't align correctly and the parts will not join as you intend. Put the two pieces you want to join next to each other (in the position they should have in the end), draw a line over the seam, separate the pieces and use the faces with the line to put the fence on, with the point where the line intersects the edge as the reference.

In case you reference with the pins/paddles (not with a mark): always reference with them from the same side.

https://www.festoolusa.com/-/media/tts/festool/festool-usa/downloads/manuals/domino_df_500_supplemental.pdf should contain everything you need to know about a domino and how to use it, it's specific for the 500 but the ideas are identical for a 700 XL.

What is the actual thickness of the sheet goods you’re using?
18 mm or 3/4” (19 mm)?
Seneca makes a metric Domiplate for 12 or 18 mm sheet goods.

Seneca has a video on their website by Halfinchshy they think explains the plate:

Here is a video Sedge did recently.

Fixed the link 7/29/20

@Df1k1 On a butt joint the reference face remains the same as when using the Domino fence, i.e., you will register the Domiplate off the outside face of the piece into which you are plunging mortises along its edge.  In cases like this, I would just double check that your edges are square (if they are not, that could explain the misalignment), and that your machine is fully seated flat on the wood especially when doing the vertical plunges. 

There're some outstanding after-market Festool inventions and accessories (the TSO guide rail square is one of them), but in my opinion, the domiplate is an unnecessary addition for an average domino joiner user.

For one thing, even if you owned all three different versions of the product, you could handle in theory only a limit of 6 different types of thicknesses in the way the accessory is designed for or to work with. But most importantly, it adds an unnecessary layer of confusion to operating the machine such as relearning how to use the machine as designed by Festool when you need to use it without the dominoplate -- say, doing a mid-panel butt joint.

The best way to master the domino joiner or any tool is to watch how people use it, use it yourself, do enough projects (and make enough mistakes along the way), and consolidate your experience. I do butt joints, mid-panel joints, offset joints, angled joints and even three-way mitre joints with the DF500 as is -- without using any after-market accessories. It shows how well the machine is designed (of course even perfect machines can be improved over time). The image shows part of a project being dry fit this week using dominoes and everything you need to make all those joints is covered in the manual.

Sage video did it. Clearly I was going off the wrong face. thanks all for the help.
Now if I could only figure out a way to get the freaking domino in the hole when it’s cut at the narrow setting!
I live in Florida but have an air-conditioned shop. I have to trim every single domino to get it into the tight fitting.  I’m wondering does anyone keep one domino tight and the rest loose or for just simplicity use the middle setting only for all dominos when assembling case goods?

Quote from: Df1k1 on July 29, 2020, 04:26 PM

Snip.

  I’m wondering does anyone keep one domino tight and the rest loose or for just simplicity use the middle setting only for all dominos when assembling case goods?

All tight dominoes on one board and on the mating board, first domino tight (for alignment purposes) and the rest mortised at the middle setting.

To reduce moisture, microwave the dominoes (in a paper bag)15 to 30 seconds (depending how many dominoes there are in each batch) before completing the assembly.

Yes.  Except for a very simple joint having both sets of mortises all tight can make for a frustrating glue-up without adding much to the strength of the joint.  So all tight on one piece, one tight and the rest middle width on the other piece is usually a good technique. 

When inserting the tenon with glue the glue usually provides some lubrication to help the fit.  For dry-fitting it is helpful to have a set of tenons with the edges and even the faces sanded a bit, mark these with big Xs to ensure they are only used for dry-fitting. 

In the video I think Sedge uses a hand plane to take the edge off the tenon, this work for the final joint because it doesn't affect the faces which control the alignment.

I searched for tips on how to created better butt joints with my DF500, and found Sedge's video. Excellent stuff. Will try out his techniques and see if this helps

Guess I’m slow too. I didn’t understand if the “offness” was up and down or side to side.

I never rely on the Domino paddles if mating narrow to narrow mortises. Just too risky. I always do narrow to medium. Tap tap and I get a perfect joint. I always use pencil marks to locate mortises. It’s simple and I don’t get reverence surfaces mixed up.