Dunbar is probably right, as this problem is usually due to poorly adjusted ball or flapper valve linkages.
Another cause of this problem, even if the ball/flapper valves are working properly, is a poorly designed low-flow toilet. The 1.6 gallon flushers on the market are not all equal. Some are better than others. By holding down the flush lever on a marginal 1.6 gallon toilet you can achieve a better flush…at the cost of using more water.