My first question is when in the fall did you slit-seed? and why did it fail?
Thatch, is the next question... If you are talking about brown mulch over the roots, then that is no problem. Fluffing it up with a slit-seeder is also no problem... Think about it.
People are so afraid of a half inch of brown fluff, but believe that 'straw' over the ground is good.
Aeration is good for compaction of heavy soils and the penetration of 'real thatch'. If you can make contact with the soil, with a broom rake, then your slit-seeder will make contact with the soil just fine.
Your patchy lawn could very easily be some fungal problem you developed last summer, possibly from over watering and over fertilizing... those are the 2 major mistakes that people do in their lawn after pre-m being in the soil when you try to seed...
Pull a plug or push in a shovel to get a cross-section look at your ground at least 4 inches deep... Post a picture of that and it will tell a lot. Everything is guess work when you look only at the surface... It's like saying, "Why does my truck rust, when I have such a nice paint job and it is always waxed?"