Filling in low spots on flat roof

2 roof problems
Standard Felt roof. There is a small flat spot on my mother's roof, about 3' by 18" wide by 1/2 deep. Is there any good filler I could use to fix this low spot?
Foam Roof My house has a few low spots where the foam is not as thick as it should be. Has anyone created a low expanding foam I can add, then topcoat like the rest of my roof?
Thank you,
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