Ivermectin won't target the encysted strongyles directly. It kills the egg laying adults. Then some of the encysted strongyles, if there are any, will emerge to take their place. Dose again to eliminate this 2nd batch. Then do the cycle again to catch any stragglers.
FEC doesn't test for encysted strongyles. Are your three turned out together? If so, need to deworm all of them on the same schedule/time so they don't pass things off one to the other.
Lavinia, Dante, George Too and Peanut
EC Support Team