Did it always do this, or have things changed recently? If it's always been marginal...it may be time for a new toilet!
There are things that can get caught in the trap of a toilet that a snake will just pass by, so there could be a comb, pencil, pen, toy,etc. trapped in there. If the water level is not proper, it won't flush well. If it has clogged rim holes, it won't clean the bowl or flush properly (a metal coat hanger can ream them out). If the siphon jet (if it has one, not all do) is clogged, it won't flush well. So, it's hard to say without being there.
If you hold the handle down to empty the tank, does it flush any better? Have you changed the flapper valve recently? It may not be the right one, and is closing too soon.