My minilab has the same issue. I confirmed the encoders perform the same way when plugged into my laptop controlling ableton live. This suggests a hardware problem.
Have you tried adjusting the settings in the midi control software?
There are several options for the encoders ie: relative mode, absolute etc. try each one if you don't know what they do, and choose whichever makes your knobs turn right.
Go thru each knob and make sure each are all set the same and make sure to save/export it to a midi pad preset 1-8. If you fail to do this and just close the program your settings won't be flashed to the controller. Also make sure you select the right saved midi preset (shift + pad 1-8) and midi channel (keyboard keys 1-16)before you start assigning midi commands in your music software.
It is a pain, but you only need to do this once. I'd recommend keeping the midi preset 1 in default so it works with Arturia's software, and save your custom preset on the other (2-8) pads.
My minilab is working perfectly with all the iPad apps and ableton 8