Yes, I suspect a mismatched antenna - alas checking matching at microwave frequencies requires specialist
test gear (VSWR meter).
The ground-plane or ground radials have a strong influence on tuning - normally for good
behaviour of a simple quarterwave antenna a set of 1/4 wave radial wires pointing downwards in a cone
works reasonably (quarter wave radials in a plane push the radiation pattern upwards, unlike a theoretical
So experimenting with the ground part of the antenna is worth perservering with. For repeatable measurements
you really need to be outside well away from obstructions and high above the ground, on in an RF test
chamber. Also be aware of overloading the receiver by being too close, sometimes there's a minimum
range as well as a maximum, but we're not really in that league with these transceivers I think.
[ Oh, the other thing, at 2.4GHz the attentuation rate in cheap thin coax is very high, you can throw
away all your extra power if driving 3m of random unbranded SMA cable - use only short runs and
only high quality cable if you have to ]