sometimes the span between the wall studs when using this tall of a baseboard, you will encounter the concave shape of the walls. Even if you do find each and every stud and even the bottom sillplate, you will still get this gap. Here is where you need to use caulking to fill the gap. No other ways unless with a very large gap, try countersinking a hole and driving one dry wall screw into the board and into the drywall, it will suck the board to the wall. Then putty over the countersunk screw head. Use this only on larg gaps between the wall studs.