I am not sure what to do about this lump. Does anyone have any
suggestions what could cause a lump like this, and how to get rid of
it?
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I hope you have an extra piece of flooring to insert. It sure sounds
like the obvious, some sort of debris caught under the flooring.

Call an new floorer if you can't get the original guy to come back. It
might a case for small claims court also. Document everything.
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Oh, BTW, is fluffy the cat missing?