advice...
There should never be any bearing problems if you follow all of the instructions. First, the BB shell width must be 69.2-70.8mm. If you have not verified that, then the problem may be obvious. If the BB shell is too wide, the wave washer will apply too much preload to the bearings.
Have you always used the same cups? It is possible that they could have have been machined to the wrong width. Cups are relatively cheap to replace.
The BB faces must also be square to the threads. I check that by tightening the BB cups until it just touches a .010 inch feeler gage, placed between the cup and BB shell. I then use .008-.012 inch feelers to search for high or low spots. If any are found, the BB shell needs facing and then remeasured for proper width.
If all that is done, all you do is grease the threads and torque the cups to 35Nm. Install the crankarms and tighten the center fixing bolt to 42Nm. Don't forget the retainer clip on the right side cup.
Last, there is always the chance of a defective crank, machined such that the distance between the bearings is wrong. One way to check this would be to install the crankarms without the wave washer. Without the wave washer, there should be axial movment in the range of about .8-2.4mm. Any less and something is wrong.
I have to assume that you know how to properly install the bearings so they are seated against the faces of the spindle and you are using the proper 6mm width bearings, not 7mm width bearings.