Let us list the things I can think of off the top of my head at 5 am in the morning
1. The switch is bad.
2. PTO needs to be gapped.
3. Engines alternator is not putting out enough volts.
4 The alternators volts are not being converted to enough volts.
5. Mower belt could be broken.
If you could give a lot more detail of the problem we could narrow this down a little.