If the symptom is the washer runs normally for most of the cycle but won't go to high speed spin and there are no fault codes, we have had this issue several times and so far have always been able to resolve it.
1). Clean out the drain trap as already suggested. Crap in there can affect the pressure sensor readings.
2. If that doesn't work, follow the Quick Diagnostics Test starting on page 6 of the document Magic-carpet posted. Once you get into the test mode, keep pressing the cancel/drain button 2 times until you advance to Step 4. This spins the motor at full speed. If the drum doesn't spin the problem is more serious, but if the drum does spin up there is no problem with the motor or drive. You can actually use this mode to drain your clothes.
I've run this diagnostic a few times and somehow it usually fixes the problem and the high speed spin works normally again during a washer cycle.
3. Last option is to replace the pressure sensor:
Wth a little dexterity, this can be done from the front of the machine. If you get to this point, let me know. I think I have some photos of the replacement process.