Advice needed - slow draining bathtub
I'm a newbie and just needed some advice.
My tenants say the bath tub drains slowly after taking a shower (ie: water is at the level of the ankles after 10 min shower). They also complained that the bathroom sink faucet had low water pressure. I had the Super snake out the bath tub drain and it drained fine and the faucet water pressure improved.
Within a week, they complained that the bath tub was draining slowly again. When I ran the shower for 5 min, I didn't think it was draining particularly slow. But I had a plumber examine it, he power snaked the bath tub drain and it was draining fine.
Within a week and a half, they complained again that the bath tub drain is slow. I had the plumber come in a second time to examine it. He power snaked it again and again it drained fine. However, the sink faucet problem returned.
In all cases, the snaking did not retrieve a clog or anything.
Is there an explanation for why this keeps happening?
Is it reasonable for a tub to drain slowly so soon after power snaking?
Each time after the snake, the plumber has assured me that it shouldn't drain slowly anymore. But within a week or so, I get a phone call from the tenants.
What is the technical definition of slow draining tub? 30 sec? 60 sec?
Would power snaking it too often cause damage to the pipes?
I would appreciate any insight for how to approach this next time they complain. I don't know what else to do, since the plumber hasn't given me an alternative to snaking it. He doesn't think that the pipes are broken and it isn't a clog in the main line.