I've had a problem with nudging blocks in the past. They would just fall off after reattaching them. So, I found that after you attach it, you press the mirror button, but don't mirror it. You then go back and do it again, and find that the object is yellow, and attached to your vehicle. If you find this confusing, just tell me.
I just use the connections button, since it disconnects/reconnects parts. If I need to move a subassembly, I’m put one part on, reattach it, then put the next part on that reattached part.