I consider myself fairly good, albeit a bit slow at trouble shooting. I had one today that was driving me nuts. Took me at least an hour of this to find.
Set up the shaper to cut some frame and panel doors. Set up the slip knife cutter for the groove first. I ran a couple of stiles and I noticed that when the board would leave the right half of the fence it would drop leaving a low spot.
I assumed the fences were not in line. Put the board up against the fence and sure enough it wasn't. But the odd thing was I couldn't align them. One was at angle to the other one. Well this made no sense! Took the fence apart assuming a wood chip lodged somewhere. That wasn't it so I planed the fence boards to make sure they were straight. Still nothing.
After a lot of wasted time I discovered my board was not straight. OK must be the jointer then. Ran a board on it and it was straight. Checked the others I had just milled they are good.
Finally I found it. I put the shaper fence back together and ran another board through and I noticed the shaper blade was not just cutting the tongue, but it was also taking a very small cut on the edge of the board. While is probably hard to picture, the results are similar to when a joiner outfeed table is set to high.
I was using slip knives and one side was out further than the other. Somehow I messed up when I installed the blades. When I fed the previously cut stile through with the shaper shut off I happened to be on the good blade so it didn't show up.
An hour or more lost because of something so simple!