I haven't tried nuking the dominos, will have to see if it helps. Unlike biscuits, dominos aren't compressed significantly (just embossed a bit), so I didn't expect them to expand much with age or humidity. But since the system is so precise, I suppose even a small swelling could make the fit very tight.
In my earlier post I forgot to mention the effects of feed rate, which Les and others noted. Info from Festool about feed rate was posted on FOG earlier (sorry, can't remember the link - anyone?). Novices almost always feed too slowly, and experienced users get confident and often feed too fast. Festool said that too fast a feed will ofteny produce a loose fit, and too slow a feed may make it too tight. They also said that a slow feed can cause the slot to slope toward the surface of the board from left to right. I suppose that's due to interaction between the rotating and oscillating motions. They have charts of ideal feed rates, but it depends on the wood so experimentation is your best guide.
As several posters here have pointed out, when there are multiple dominos in the same joint (i.e. the same two pieces of wood), very small misalignments can make it difficult to impossible to push the joint together. This is the same issue as with classical round dowels, where the holes have to be exactly aligned and straight, but the domino is more precise than any dowel cutter I ever tried.
The most common advice, as from several posts here, is to use the wider settings on the cutter or to manually widen the slots a bit. I use the tight setting on all the slots in one piece and in exactly one slot in the second piece (to assure alignment of the parts). Then I use wider settings in all the other slots in the second piece. The only trick is to remember that you want to align the centers, not the edges, of the slots so that the widening can do its job.
Multiple slots also have to align along their long axis. That is, they need to be parallel to the face of the board and all the same distance from it. Misalignment that way is most often due to user error, especially not holding the fence tight against the wood or tilting the machine while you feed. But there are also some more subtle possibilities.
The above-mentioned sloping due to slow feed is one very subtle kind of misalignment.
Some people have reported that the depth setting on their domino slips during use. I haven't had that problem, but you might want to check. This problem is easy to detect, as you can see the setting change on the fence scale. If your fence slips, you should send the machine in for repair.
You have to be especially careful if there are any rabbets, dados, slots, moldings, or other shapes on the edge of the pieces to be joined. The spring-loaded pins can catch on these features and pull the machine off the desired alignment. I've had this happen several times with dados for panelled doors. The best techinque is to put the fence against the piece with the machine far enough from the edge that the pins don't touch. Then slide the machine up to the edge while keeping the fence tight to the piece. Watch where the pins go as you do this, and make certain they retract completely.
Hope this helps
