Laminate flooring and level floors

I'm installing a laminate floor on a concrete slab - using the recommended underlayment. I have found areas where the floor is not level and the laminate sinks a little when you stand on it. These areas are fairly gradual so they weren't really noticeable to the eye before I put down the flooring - especially if you're not looking for them.
Anyway, I hadn't finished so I was able to take up some of the laminate and I attempted to fill in the areas with a patching filler. This has not been very successful. One area still 'sinks' and I'm finding it very difficult to make the patching stuff level. It just ends up kind of lumpy from sticking to the spreader as I try to smooth it out.
Does anybody have any hints or ideas on how to get around this? It may not sound like it but usually I can manage DIY projects - but this one is just wearing me down.
Thanks.
Paul
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D'Olier wrote:

This is a perfect example of why proper preperation is essential. Checking the slab witha straightedge and using the correct materials to fill prior to installation of any finish flooring.
Guess you could remove and start over.
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One reason to buy a floor like Mannington that can be picked up and put back down as many times as you need without ruining the locking mechanism.

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If you are putting a floating floor down this won't help. Mine is glue down.

The flooring is 4" wide. I would work 4 rows at a time. I cut some plywood squares 10' X 10". when there were low spots I put the plywood square over the flooring then put a River rock on top of that for weight. You can see the low spots when your laying the floor or push down and feel them. One thing to watch out for is don't cove a joint or you might stack a joint. My flooring was 3/8" thick, engineered. They do make some self leveling compounds If you are doing a floating floor that might be ticket.
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