Problem with toilet flush

We have installed 2 new American Standard toilets in our new home. One works perfectly. The other...not so great. The flushing seems so inconsistant. One time it will be strong, the next pretty weak. The waste seems to clear every flush, just not the toilet paper (not much paper is being used!) Just looking for some things to try before I have to call my plumber back out. Thanks!

Call the plumber and let hin check it out. Has it done this from the very start, since it was installed? Was the old one doing it also?

Take the lids off both of them. See if they fill up to the same height. Then flush both and see if they empty the same amount of water into the bowl before the flapper comes down. If everything is equal, try plunging. If that doesn't solve it, you may have a drain or vent problem.