Sometimes the starter motor will not engage with the ring gear on the flywheel. You just hear the motor spin but nothing happens.

The most common problem is simply that the battery is either getting flat, is old and tired or, occasionally, faulty.

First check and clean all the connections, from starter to battery, every one.

Try your normal battery again. If all's OK, great. If not continue.

Now try another battery. Preferably a big, known good one. Like your car's. Try connecting the positive jumper lead direct to the positive starter terminal (the insulated one) and short the negative to the body of the starter motor. If it engages you know the motor is good. If not you'll need to do some more investigating inside.

Now try the jumper leads into the battery connector. A purpose built connector with short leads you can clip the jumpers onto helps here. Make sure you have a really good connection to the plug terminals or you're wasting your time. If the starter still works OK then you know the battery is the problem.

If the motor wont engage then it looks like the wiring or relay (or for mosquito owners, the switch) might be the problem.