Why won't my grass fill in?

I have a blend of p. rye, ky blue and fine fescue. Back when my lawn was seeded in the Spring a lot of run off created some large erosion gullies and lots of smaller ones that are hidden by the grass now.
On side note I filled, seeded and burlapped the large gullies but repeated deluges have removed about half the soil I laid, so I must wait till the Spring to fix it. I think I will use sod instead. If anyone feels the new soil under burlap should have withstood the heavy rains and that I must have laid it wrong I welcome any tips. Most books say, lay straw or burlap or some of that new green stuff.
My real question is that I notice when I mow the smaller gullies still are free of grass even though they are not very deep (1/2 inch at most and an 1-2 inches in width). The grass around them is thick and green. Given the grass is growing now why do these areas not fill in?
Thanks for any advice.
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***snip***

My guess would be that the topsoil is eroded away and the only dirt left may not support the new grass.
Chris
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Interesting. Can I infer from this that the grass that's there will not dig below the loam, because the soil is not suitable?
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I read a lawn care book from Ortho, and it says that we need to mow the lawn often when the grass is actively growing in order to encourage it to grow out instead of growing tall. If yours is cool season grass, the "active-growing" season is spring and fall. If this is the case, you may need to mow the grass often to keep the grass low enough (even twice a week). If my memory is right, you are better off doing this (mowing often) in spring (rather than fall). Unfortunately, I don't have the book with me right now, and I cannot verify if fall is OK to mow often or not.
I don't remember how low you should mow the grass. I only remember that you need to mow often. You may need to check this out in the book store.
I haven't tried this yet. I intend to do just that in next spring (or may be this fall).
Hope this helps.
Jay Chan
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