Objectionable Odor

I have an ice cream shop and am dealing with a sewer gas odor. I have been in the shop for two years. The landlord has covered the vents on the roof from where the sewer gas emits. They have also put inserts on the grease traps behind my shop. After all of this was done it seems to have helped somewhat. However, after a new tenant did a build-out next door to me I noticed the smell became more prominent especially in my bathroom.
Any ideas what I can check?
Thanks.

When you say he put "covers" on the vents, did he cap them?
Is there an exhaust fan in the bathroom? Where does it exhaust to? - could air be coming from the roof in though it bringing the gas with it?

Have the toilets been pulled and reset? If the wax ring is compromised you can get gas from there and not be able to tell where it is coming from. I was called for an odor problem that a plumber worked on for months trying to find. It ended up being a C/O cap that was behind a toilet that the cap had fallen off and no one noticed because things were so tight. 1 call, odor gone, I became their new plumber.