Both the ESC and Servo have a three wire connector to the receiver, those three wires have to be in the right direction in order to work. On most receivers the black wire on the plug should go to the outside. Some connectors have a tab that only allows you to plug it in one way, but others can be turned around backwards. However I suspect you could have motor trouble if you say that the ESC seems to be acting normal. Try giving it a little push while you are giving it throttle, if it goes or tries to go you may have burned the brushes.

It could be as simple as some debris got in the motor. It is a worthy investment to pick up some motor cleaner and bushing/bearing lube. You may or may not get this motor back to peak performance, but it will keep any new motor you might buy in peak condition longer.