If your toilet has a lever flush system, there's a chance the lever has disconnected from the flapper. The lever is typically attached to the flapper by a loose chain.

If this piece of tubing comes loose or comes out of the overflow tube, it won’t squirt water into the overflow tube to refill the bowl after you flush. This will cause a weak flush.

Regular toilet bowl cleanings can help with water flow and prevent mineral deposits ... Too much slack on the chain may be the problem, so you may need to adjust the chain to make it less loose.