I thought I was the only person with this problem! My hair has a very slight wave in the front (I've been natural for appx. 4 years) and I can't even think about running a comb or denman through it. It will be completely straight. I usually do a flat twist in the front and after a day or two I can take the flat twist out and my hair will blend with the rest.
I cut the front of my hair a lot, also. Because its so straight, it looks longer, so I give myself a long bang when I get tired of having to hide the straight pieces.
I've tried beer rinses, protein treatments, twist outs, roller sets, scrunching with flaxseed get after washing with no manipulation....you name it, i've researched it and tried it. Unfortunately, that's just the way our hair grows.