If you fail to regularly remove the grit that gets in the seals, it grinds the lower most part of the circular to an off-round shape so it won't fit down snugly in the seals any more. The only thing you can do is replace it but guess what you can't buy the individual bits, you have to buy the whole assembly.
However all you need to do is un-bolt the front part of the gate valve assembly leaving it fixed to the machine, replace the gate part along with the new seals (might as well) and then bolt it back together again. I used to have quite a few unused halves of these assemblies and old seals chucked in a box in the shed!
You should try to make it a habit at the end of every day to wash the waste tank out with a hosepipe, keep the gate open and stick the hose in there to wash the grit out from between the seals. A bit of silicone lubricant on the gate and also put onto the spindle from above will help keep it operating smoothly.