Bottle Feeding Problems

Hello Mummies, my child is about 2.5months now & recently we have been having trouble giving him bottle feed. He will just put the teat in the mouth or tries to move his head away & sometimes gag when the teat is inside the mouth. Other times he will continue sucking but sometimes a feed takes an hour to finish. Does anyone face the same problem? Should i change to another bottle? Im currently using the Avent PPSU natural. What bottle recommendations is there?