this is expected, even on petrol...if you use the same gear to climb you'll get good pickup, but if u increase the gear at start of climb or even during climb the pickup dies.
if you slow down and have to pick up again, of course you'll notice a change in engine response.
the key is...during a climb you use your momentum as well as the engine power to overcome the extra workload.
all this is exacerbated on CNG.
try this...switch to petrol about 100 yards before the climb, build a good speed, and maintain the gear during climb.