Might be worth checking the installation. The interwebs are littered with accounts of under-performing wingtip antennas, that are later found to be installed improperly. Grounding (poor, or wrong element grounded), coax polarity, coax routing, *other* wire routing, other stuff 'in the neighborhood', etc etc.
The coax and its connectors can have issues, also, of course.
Just a couple of thoughts...