The lack of upshift can be caused by low fluid level or foaming caused by high fluid level. If the problem was caused by foaming it would go away after setting for a while.
I have also seen problems occur when the the filter doesn't seal properly to the valve body (causing air to be sucked in during hard driving. You should verify the oil level again making sure you have enough LOOK AT BOTH SIDES OF THE STICK. One side often is much higher than the other. Go by the low side.
Bosch Master, ASE Master, L1
33 years MB technician