Two most likely reasons:
1. It needs lubrication. A manual toilet is working machinery with parts
that rub against each other. When they leave the factory the pump
is slathered with a thick teflon or silicon grease, but it doesn't last forever...typical just a season before the toilet starts to become increasingly stiff as seals
, o-rings etc start to rub against the inside of the pump cylinder without anything to "cushion" em. You can join those who opt for the never ending job of flushing
down the toilet every couple of weeks (a tablespoon will last just as long as whole bottle 'cuz it all gets washed out in few flushes)...or you you can buy a tube of thick teflon grease (SupeLube has the best one, Ace Hardware
and Home Depot carry it)...open up the top of the pump, or lift
it off the base to stick the nozzle on the tube into it and give it healthy squirt. Put the pump back together, pump a few times to spread the grease all over the inside of the pump and you're done till next spring.
2. If you're also experiencing any burping, bubbling or back flow and flushes that don't want to go down, that's a strong indication that you're pumping against a blocked holding tank
vent. The two most common locations for a vent blockage are the vent thru-hull and the other end of the vent line--that end of the hose and the vent fitting on the tank. Start by cleaning
out the thru-hull...use a screwdriver blade, ice pick--whatever works. If that doesn't result in a spew out the vent, you'll need to relieve the pressure before removing the vent line from the tank to clean them out...so open the deck
pumpout fitting VERY CAREFULLY with a hose at the ready. Be sure you're UPwind of it! Scrape out that end of the vent line and the vent fitting on the tank...replace the vent line.