We had a "bumpy lawn" and tried everything. You'll probably end up needing to 'top-dress' the areas affected by these 'bumps'. Top dressing is a term used to describe a mixture of washed sand (clean) and, perhaps, some compost.
You'll need to get a bunch of it, broadcast it over the affected area, rake it out, fill all the voids by any means necessary, broom the sand sitting on top into the void spaces between the grass, and then water it all in.
The grass will eventually come through the top dressing and you'll have a nice smooth lawn.
Another option is to water it really well, then pack down the affected area with a lawn roller. But this can backfire.