try finding the piece that sticking mostly off the frag and away from the main colony.... I usually only frag those.... lessens the chance of messing with other polyps.
BTW- My SVDMs are finally a colony again after 3-4 tries, I and won't frag. But will place an empty frag right beside the colony to allow it to grow on that.
also they don't need to be fragged to just melt on you..... they sometimes do that on their own.