When toilets are hard to flush, the underlying problem is typically an issue with a clogged pipe, low water in the tank or the flapper valve closing too soon. Typically these problems can be solved without having to call in a plumber.
If the toilet's drainage pipes are clogged, water and solids back up into the toilet and make it hard to flush. To determine if this is the problem, fill a large bowl or bucket with water and pour it into the toilet without flushing. If the inflow of water causes the toilet to flush normally, there is no clog in the pipes. If there is a clog, try using a plunger on the toilet to see if it releases the clog. If it doesn't, run a plumber's snake through the outflow pipes to break up the clog.
If clogged pipes aren't causing the flushing problem, lift the top off the toilet tank to check the water level. The rubber ball float that rests on top of the water determines the tank's water level. If the float's level is set too low, not enough water is released to complete the flush. Adjust the float's level manually to increase the water level in the tank.
If the problem is caused by the flapper valve closing too soon, check the length of the chain attached to the valve. There should only be 1/2 inch of slack on the chain; if there is more, adjust the tension on the chain. If none of these solutions work, calling a plumber may be necessary.