Ok, I'll chip in here. Usually when you use spacers, filters, extension tubes, etc... over time aluminium dust gets into the threads and it makes them gritty. You need to clean them and oil them with a bit of WD40 or other on a cotton bud so you can 'wet' the threads. Doesn't have to dribble. Clean (aluminium dust free) and 'wet' is good enough.
Now keep in mind not to screw them too hard. They're fine threads. Also when the rig's spent the night out there is absolutely no way you're going to unscrew anything. So you wait all your spacers warm up, take them in front of the heater the following morning then you'll see they'll come loose by just turning by hand like you screwed them in the first place. Do not use tools, pliers or other to get a better grip. You'll break something. If it's stuck, warm it up until it gets lose.
