Had a pipe put in under my lawn. Grass refuses to grow/dies where the pipe is. How can I fix this?

I had a gas heater put in for my pool. The pipe that connects the heater to my gas was buried several feet under. It's been 2 months and grass won't grow in several patches. I've tried replacing the soil and reseeding and the seed either won't take or will take then die shortly after. Help!
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