Have you moved your extending length too high? You definitely have to work into the extender length bit by bit. When you start, you shouldn’t be stretching to your EL yet. It takes time too work your way up, that may be why it’s slipping.
Time is similar you just need too increase the time little by little, and if your extending at a proper length for you at the time, you should be able to work to getting your time up. Also try warming up, use heat and stretch before going into the extender.
I’ve moved onto a routine, but when I used an extender I never got over 5 hours, just because of lack of free time when I could use my extender. I would similarly wear it for about 1.5 hours and then get the circulation going again before starting again. I found it very uncomfortable wearing out and about, trying to work and walk with it on so I only ever used it at home.
I have the 4x extender
Start: Nov. 2017 - FL: 3.5", FG: 4", BPEL: 6.813, EL: 6.25", MSEG: 4.75"
Current: Feb. 2018 - FL: 4.75", FG: 4.625", BPEL: 7.375", EL: 6.8125", MSEG: 5.25"
Short term: NBPEL: 7"x5.25" - Ideal: NBPEL: 7.5"x5.5" - Dream: NBPEL:8"x6"