firstly - dont add more top coats - acryllic spray can take a lot of time to fully cure at the best of times - this wont help, I've tried it...

wokka's suggestion of sanding back to the primer and adding more is not a bad idea - perhaps it has shrunk as it cured.

alternatively - sand back and use a product like timbermate - like a wood putty, water soluble - and make like a paste with it by adding water to an amount. Rub this in , let it dry , then sand back - lots of guys have used it here, have a quick search and look at what they have done