flashfireopal

I've noticed this, but very rarely. Sometimes when I'm trying to add an engine to the ship and there are 8 slots open and the engine requires 8 slots it won't let me. And selecting another part then reselecting the engine doesn't help. I haven't encountered that yet, myself, and I assure you that I've taken maximum advantage of many designs.<br

The red background is a warning that you can not add the component you currently have selected to the hull without going over the space limit. So, in your case, you can't add another engine. Try switching to another object, say whatever life support you have, and see if the slider changes.