You may even be able to dislodge the relay contacts without touching it directly. When mine occasionally does this I can get it to dislodge itself by giving a rather sharp rap on the right side of the relay box near the very back (closest to the seat). And I do mean a fairly sharp whack, perhaps with the back of a screwdriver.
The relay may or may not need to be replaced, even if you're successful in dislodging it this way (or by tapping on top of it directly, inside the relay box).
Lastly, this is often the result of a battery that needs to be replaced, not just charged. If you charge up your battery and this keeps happening, you might look to replacing the battery before the relay if the battery is old, or has been subject to periods of disuse, etc. Just because it tests at a good resting voltage doesn't mean it's able to deliver a healthy current to the starter. Sticky starter relay is one of the prime signs that it isn't.
