Mine does the exact same thing, it drives me nuts. The parts just seem to randomly lose focus. I usually have to click the selection tool again and re-select the part to continue establishing relationships.. I've been tempted to try to go back to version 16.. I'm sure my company wouldn't be too...
When we upgraded to version 17 at my company, I noticed that in assemblies now, flashfit is the default command when I place a part into an assembly.
I don't necessarily have a problem with flashfit, and I am sure that if I gave it a chance it *could* potentially reduce some steps in my...