Hi got a shower tray that drains slowly causing the water to build up in it. Its brand new so not blocked. It came with its own trap and flexi conecter which is smaller then the 40mm waste pipe its connected to. When I fillled it with water to test for leaks I found the water did not drain. However when i blow in to the waste pipe for the sink wich is further about 4 foot from the shower it suddenly pulls through and drains. Any ideas on what could be causing this and anyway to rectify the problem. The waste pipe isnt flowing up hill.