Why do insulin injection lumps happen?

I've noticed some small lumps under my skin in areas where I regularly inject insulin.

I try to rotate spots, but sometimes I end up using the same general areas because they're easier and less uncomfortable. I was wondering why these lumps develop and whether they can actually affect how insulin works.

Do people usually wait until the lumps disappear before using that area again? Also, how do you remember to rotate injection sites properly?