Although staying with a 'difficult' partner can help you overcome a problem - I've actually found it better if you swap around a lot and, through training with others, find why it was different with this other partner. You can then go back to your original partner and retrain with them. The only danger is if people actively avoid each other - but that is why at times I specifically suggest two people train together.
In weapon work we always used to stay with the same partner so we could blend effectively with them when it came to more complicated stuff. I think this is useful, but in my classes I've found that, because there are a lot of low grades, swapping speeds the progression up enormously because they can learn alot from each other, and the more knowledgeable people spread their knowledge around as well.