Ok! Let's get one thing right, straight away. You do not get a level lawn by using a roller. Using a roller, simply gives you hard bumps and hard hollows - and all the bits in between.
In fact, unless you are going to carry out all of the other operations necessary for a first class lawn, you can get rid of the roller altogether - although I might concede that it is useful for a light run over before the first cut after the winter.
In actual fact, a lawn rarely if ever, 'grows' a bump! There are no logical reasons for a lawn to rise up in places to provide you with a bump! What generally (always) happens, is that the lawn sinks over time - sometimes taking a few weeks as is the case with new lawns, or sometimes over a period of several years, as soil naturally sinks in places as a result of natural earth movement (rare) and underground roots and organic matter decomposing.
Most lawn bumps and hollows are cause by natural soil settlement or erosion over years in the case of mature lawns, and over just a few weeks with a badly prepared surface for new lawns.