We gutted a house to the brick and put in new infrastructure. None of the old plumbing remained. Bought 3 expensive brand newtoilets. They were installed according to the instructions on the box (not using a 'horn' gasket). The shorter flush barely flushes pee, the longer one is only a smidge longer than the short one and is also inadequate. The only way to get a proper flush is to hold both buttons down and once and keep them pushed for at least 5 seconds, overriding the flush so it is much longer. Unless we do this, the toilet water remains dirty and the pans get a thick yellow slime within one day. One toilet has limescale in the bottom already. These toilets were installed less than 1 year ago.

We've ascertained it's not the installation (plumber reinstalled 1 and it's still bad). I doubt it's the toilets themselves because they're all behaving exactly the same (weak, short flush, accumulating bacteria). Could this be the venting system? That was newly installed too.

Thank you for any advice.

Is any of the drain piping for the toilets visible from below the floor?

Because they are all behaving exactly the same, I'd say it IS the toilets. How about a name and model?

If there is NO venting system, then that MIGHT be the cause, but even then not likely.