@Roseann Koefoed Roaches are very difficult to get rid of. You can do treatments every 10-days, but that's overkill. It sounds like you've treated heavily. I'd put the building on a monthly treatment plan with a responsive pest vendor.
I will say every pest vendor I've dealt with have recommended against fogging. I'm not sure if your pest vendor is well reviewed. If you're still dealing with them, maybe look into another vendor.
I've gotten a few quotes recently for monthly service, 12-month contract for around $15/unit.
Another thing that will help is constant communication with your tenants. Let them know the next steps after treatment. "we're going to check these glue traps a few days after treatment to monitor progress" " I'll send a cleaning vendor out if you're still seeing them, dont worry, I've got the bill cause I value you as a tenant"
Retaining these folks is cheaper than running ads, lease commission, and application fees.
I have a great recommendation for a pest vendor in Chicagoland area if you're interested.