P98 is usually very reliable - I can't remember the last one I had for repair where 'the fault' wasn't caused by the user.
First of all you can eliminate the possibility of a faulty motor by connecting it directly to the battery in each direction in turn to make sure it actually does run both ways.
Then make sure you are supplying it with at least 5v via the receiver. Less than that and it might not switch from forward to reverse. Do note the instructions which tell you there's a delay between selecting forward and then reverse - you must centre the throttle stick and leave it there momentarily before changing direction. If all that fails and you're satisfied with the wiring connections you've made then it's probably faulty. Send it back to Comp-Shop for checking/repairing/replacement. The normal warranty T&C's will apply.
Dave M