Tray 5 of my Invisalign fits loosely, advice?

Hey Eli 
It will be fine as long as it fits correctly. There is a misapprehension that all the trays do the same thing. It is designed by the doctor and some will feel like they are doing more than others. 
Dr Luke

PS if you don't wear them I guarantee it won't work. 

Hi there - sometimes moving to a new aligner doesn't bring any discomfort to the patient.  Depends on the type of movement that is occuring on that tray.  But if you're concerned - go see your dentist and see if you tracking.  Moving to aligner 6 when aligner 5 didn't fit will just cause more problems and time in the future.  Good luck.