Here are some toilet problems that can cause weak flushes:
If there is an issue with the flush valve or flapper, it may drastically reduce the amount of water that gets put into the bowl with each flush.
If there is an issue with the fill valve, it can also result in a tank water level that is too low and cause a weak flush.
Finally, if there is a partial stoppage in the toilet bowl trap or the siphon jet, this will also cause a sluggish flush on a toilet.
If you are having issues with poor flushing performance on a low-flow toilet (1.28 gpm), one of these may be the issue, but it could also be because the toilet was poorly designed in the first place. If the toilet used to flush great, it can probably be restored. If the toilet always has had a weak flush, replacing that toilet may be the best option. There are some really good low-flow toilets on the market (Kohler, Toto, American Standard all make some good models). There are also pressure-assisted flush toilets that can be installed as well if needed.