Roundup against dandelions, it went away the first time, but was immune the second and third time, how can that be?

Roundup against dandelions, it went away the first time, but was immune the second time, how can that be? The first time I sprayed it, the dandelion disappeared, dried up, turned brown, then reappeared in the same place, lush green, and spraying it with Roundup the second and third time had no effect…;( Is there…