Yellowing of gelcoat is a sure sign that either your wax has oxidized; or, that you didnt apply a 'good' wax; or, tannins or rust staining has now deeply occurred in the gel.
If tannins/iron, wash the affected area with oxalic acid (wood bleach) then apply wax.
If the 'old' wax has oxidized, then use a caustic (such as TSP -trisodiumphosphate) to extract/strip the old dead wax from the 'pores' ... then rewax.
Collinite 'Fleet Wax' is probably the longest lasting
When waxing dont 'smear-on then smear-off'. Instead 'push' the wax INTO the gel using a bare wet hand. In 2 ft. by 2ft 'squares', apply a little water to a clean palm and a bit of wax. Keep pushing until the gel brilliantly shines, using a few drops of water as necessary. THEN, smear on a small amount of wax and use a knobby foam pad mounted on a variable speed autobodyshop polisher to 'buff'.
For relatively new gelcoat yoiu might want to consider to firstly lightly 'buff-out' using 3M Finese-it™ followed by 3M Perfect-it™ (using a knobby foam pad and autobodyshop polisher).... followed by waxing.
Dont apply 'wax substitutes' .... .