If you need to hold the buttons down for a few seconds to get a complete flush, the valve may not be working correctly.
A toilet valve is designed to flap completely open, and then stay vertical long enough for the tank to clear nearly all the water (get a good complete flush), and then slow hinge down and close off the opening. If the lever is misadjusted it won't go completely vertical, and if the valve is failing it may close too quickly or maybe not seal after it's closed.
Is what you are saying for a traditional toilet or is this the same for a 2 button low flush?
Usually the push rods on dual flush need adjustments if the toilet is not flushing well.
I think mine works fine. I hold it just barely a second. As explained before, traditional toilets, once you hit the flush lever, generally the ball **** valve stays up for a couple of seconds. Holding the lever or not, does not really change much.
On this low flush I have it is more like, you press and it flushes, the second you remove your finger from the button, it stops instantly. So I hold it just a fraction of a second more than a regular toilet, and it works great.
Right now it works fine, If I fiddle with it trying to adjust something, I could easily F-up something, and I have too many other things to do.
Definitely, some day in the future, it will need fixing or tuning and then I will be back on here asking how these toilets work.