I understand, and in that case it's very understandable that I can't reproduce it either, not knowing your scenario at all.
Personally, when I have problems like that which I can't quite put my fingers on, I use the narrowing down process. First I make a copy of my form, preferably to a new project so I can experiment at will. After every step of the following list of steps, I check if the problem is still reproduced.
- I try to remove all the data dependency, so the form will run without an active connection to a database or webservice and it can run standalone.
- I start removing controls that don't seem related to the problem, keeping in mind that this exact layout I use in this particular form might also be a clue to the failure. Most of the time I'm doing this I manually edit the designer generated code, first cutting form.Controls.Add() lines for controls and then try running. If it stops failing, I paste back that control(s) and try further narrowing. If it still fails, I cut a few more form.Controls.Add() until I reach down to the bare minimum. During this process, you hopefully find the spot that matters. When you do, still further narrowing might be possible for example if the problematic control is a container etc.
In which order you perform these steps is up to you and your preferences. The procedure is always the same though. Narrowing and more narrowing until you find the problematic control and hopefully the problem.
Directly editing the designer generated code like this has the advantage that you know exactly what changed, and there are no related changes. Make sure the form designer is not open while you make these changes as designer updates may interfere with what you are trying to accomplish.
Hope this helps,