First check the forward tank vent for a blockage
If the air in the tank displaced by "incoming" can't get out, the tank pressurizes, creating backpressure. Enough backpressure and the toilet can't flush against it. Easiest way to check is, verrrry carefully loosen the deck pumpout cap...have a firm grip on it, and have the hose handy...'cuz if your tank is so pressurized that it's preventing the toilet from flushing, you'll have an eruption. If the vent IS blocked, the first place to look is the vent through-hull. If the tank vent isn't the problem... The most common cause of sluggish (or no) discharge in electric toilets is low voltage. Put a volt meter on WHILE FLUSHING...if it reads less than the nominal voltage (12, 24 or 32, depending upon the boat's electrical system), check the connections for corrosion...check the battery itself to see if it's getting weak...wire size to see whether it's adequate for the distance. If the problem isn't electrical, check the joker valve...that's the first place clogs occur. If it's clean, try flushing it while you have the discharge hose off (into a pan, of course). If the problem is a clog downstream of the toilet, it'll flush just fine. If it still won't flush, the problem is in the toilet itself...the discharge impeller may be shot...or, depending upon make/model/age, there are other possibilities that can be solved with a rebuild kit. Or, depending upon make/model/age, you may be better off just replacing the toilet.
