how can I fix my toilet?

My toilet will not flush properly. Everytime we use the toilet we have to keep pushing down on the handle repeatedly (as if to 'pump' the toilet) which will then sometimes work and after a few attemts the toilet finally releases enough water to flush but most times it just dies down and only a little water comes out!!
It is a heavy lidded toilet the top is not too close to the toilet.

Can we fix this ourselves or do need to call plumber out?