The thing with motors is they produce a certain amount of horsepower, and not any more. They are usually sized to do their job, and they don't usually wear out and start lagging or act weak.
There could be many problems, but if a motor is trying to move the load, and the load does not move, perhaps the load is stuck in some way. Or perhaps the connections to the motor are not strong and tight.
For many mechanics, changing a motor involves a lot more than just taking it out and putting another back in. While they do that, if they need to they adjust things so the load moves easier, they tighten connections, and they test the motors too. Some will change first, charge you for a new motor and installation, then work on why it does not work.
It's never an easy job finding out why things do not work, but very often the problem is not the mover itself, but the parts that allow the mover to move the load.
A mobile mechanic with a good reputation will have ways to analyse your problem first, then fix it as easy as he can.