When you say "dig" I'm assuming your main house drain is buried, or enters the concrete at the low point. If you have lead joints (cast-iron)piping, then get a plumber out there to correct the section that your having problems with. If it's PVC, then you may be able to add a hanger where the belly is occuring. Normal pitch on 3 to 4" diameter pipe is 1/4" per foot.

It's PVC, thanks for your response. I appreciate it