I used to have that problem too, until I got a new unit. The pumps are far superior from earlier models, as are the sealing wires. Canisters now seal better because the suction is better, but the seal does goes bad (hard?) on some old canisters.
I have one of the old big ones that still refuses to stay sealed. It has an old red connection that I don't even have an adapter for anymore so I use the mason jar sealer to seal it. I hate giving it up so I'm going to try using some wet emery cloth on the rim and a little cooking oil on the seal to see if I seals, though it actually may be in the top connection where the leak is.
Support bacteria. It's the only culture some people have.