Sounds like possible cross contamination, or capping station not doing it's job properly. I'd soak the heads, do a couple really good cleanings, see if it helps. If not, next step would be replacing the pads on the capping station or the minimum, take them out and put them back in, making sure they are tucked in tightly and securely. If still bad, look at all the lines around the capping station and make sure none are nicked at all, and if not, then maybe consider a new capping station pump. Pretty cheap and they're known to not last very long.