Paul,
Using FF 3.70
If a combobox's items in the custom properties dialog are the only changes made, compiling doesn't include them.
Another change is required. I've tried the following but other modifications may also force a correct recompilation:
1) Edit code
2) Edit the control name
That's odd because I fixed that bug in 3.70. I will have to look again. :-(
That's what I thought too.