When you flush to cistern, does the water level rise quite high in the pan..?
If it does, that would indicate a blockage.
If it does'nt ... are you sure the cistern is holding the maximum amount of water to complete a full flush.
The more modern low level cisterns only hold 6 litres, which is often only barely enough to clear the pan.
Another possible way of increasing the volume of water used in a flush, is to check and see if your flush valve is adjustable.
Without seeing what valve you have fitted, it would be difficult to advise.
A picture would help, and I'm sure others would give advice.