On a number of occasions I have seen pilots have trouble getting into their harness after take off.
Most of us set up a harness for comfort suspended form rafters etc this works well for comfort but not for testing the ability to get into the harness.
With the exception of Advance Harnesses (which are very different to most others) the problem is usually that shoulder straps are too tight and the leg straps are too loose.
What works for me is adjusting the shoulder straps so that the front of the seat is level with the backs of my knees when standing upright and then do up the leg straps as low as possible and as tight as possible (as per the attached video capture - from Suppair)
Its best to test with the aid of a couple of strong pilots on the ground lifting you from the risers.
Hope this helps?

Nasher
PS I seem to remember doing this with Humphrey at a busy Rhossili day with his new harness, he promptly took off and flew for 4 hours, leaving me to do the coaching that day - thanks mate! (it does prove how comfortable it was)