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I've rebuilt a tons of induction motors. It could be worn or dirty bearings. If the bearings are worn the rotor will not be perfectly centered in the stator. That could cause it to run slow or even lock up. There should be no up or down play between the shaft and the bearings. It could also be dirty or dry bearings that are preventing the motor from reaching speed necessary to kick off the centrifical switch. It could also be a sluggish centrifical switch mechanism. It could also be a bad winding but that's not too common. I'd completely disassemble the motor and clean everything with non flammable electric motor degreaser. It won't hurt the windings. Now make sure the centrifical switch assembly has good clean contacts and moves in and out easily.
Make sure the bearings get a good blast of cleaner. After the cleaner dries, oil them up with a ISO68 non detergent oil. (synthetic is preferable but you can also use 3 in 1 if you get the grade for motors). Make sure any wicking surrounding the bearing also gets a shot of oil. Polish the rotor shaft with 600 grit wet or dry sandpaper then coat it with a thin film of oil. Re-assemble making sure both end caps are tightened flush with the end of the case or you'll get some bearing bind. Does it work OK now?
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