Plumbing?

The water takes some time to run away from my bath, during showering (not a power shower). The trap is clear as I've had this off to check and cleaned it. The pipe (plastic) has had a flexible drain rod down it which does not appear to be blocked. The bath plumbing runs into the soil stack which is about 10 feet away with a couple of bends.
If the bath is used and then emptied the flow is much better but for a short time when it returns to slow runaway. I have used Mr Muscle etc and an Industrial Unblocker (Hair and Grease) Drain Opener from Grahams but all have failed. Any suggestions?