I'd be tempted to let the agents cleaner do it, and if
he removes ALL the stains you will refund the money.
If he fails it's the agents expense not yours.
Whats happened in the 3 weeks since cleaning, have workmen been in
and perhaps messed it up. Go and have a look, and put
the above proposal to the owner.
It does feel like you personally are being criticised, but
we are all human, you will get the odd problem. As long
as they are few and far between.