It is tough to draw a conclusion without first seeing your shot. However, merely speculating, the off-hand might be the culprit, just as you suspected.
If your shooting hand has its fingers spread wide, with the rock resting on the pads of your fingers, and you're certain you snap your wrist and follow through, spin should occur naturally. One of the few ways to prevent
spin would be if your left hand were somehow counteracting the work of the right (assuming your right is your strong hand).
Again, this is all speculative without a video, but I may suggest experimenting by holding your left hand completely straight during some practice shooting, as if you were going to use it to karate chop. Like this:
Try firing some jumpers with your off-hand straight in that manner, without gripping the ball with that hand. My little brother went the extra mile and made himself a paper plate contraption with finger slots, so his left hand could hold the plate like a glove and would thus be unable to use the off-hand as a means of propulsion. It more or less forced him to only shoot with his strong hand, with the lefty simply there for the sake of security.
If your shot still
doesn't have spin after that, I may need a video.