During switching operations, after a while, some piece of rolling stock will suddenly not couple to anything. At all. Sometimes it's a hopper, some times it's a boxcar. Sometimes it's a piece of rolling stock that came with a route and sometimes it's user-made. Using different cars or engines to couple to it doesn't work either. And in certain track design situations, this renders the entire switching scenario a failure.
One thing I've noticed it that this seems to happen following an inexplicable inability to click on the couplers to separate them. Normally when we mouse over couplers between two cars, the cursor changes to a "separate these couplers" symbol, so to speak. But after a while, again for no reason, this stops happening and I must use the bottom slide bar to locate the two specific car numbers attached to my locomotive and separate them there. I can't say with 100% positivity, but it seems to happen in tandem with the coupling issue which makes me think they're related.
This has been happening for a very long time and I'm really fed up with it. Any ideas what's going on and how to fix it?