The free spins won't necessarily guarantee us wins. And it's very typical for a free spins winner to expect that there is something that will come out of it once these spins have been spent. But the usual outcome is that all of it will just be spent and gone.
I noticed that when one is playing on the same slots for too long, it gets harder to get a good win, and most of the time, when the free spin is triggered, it often gives a crappy result.
Could it be because of the algorithm that it has got and the longer that we play slots, the system detects that we're enjoying it and results do always come in favor of the casinos? I don't know but that's the usual guess and thought that we might have if we're a regular slots player.