Think you may have helped me solve a similar problem. Moved in to new house earlier in year and have 2 showers upstairs 1 getting bad smell now and again other is fine. Due to this thread had a look and the one that smells sure enough has a part missing in the trap looks like it’s a one way water goes through but seals if no water going through creating a seal. Can I buy this part seperate or will I need a whole new trap? Anyone know the make and or supplier of this type of trap. Pic of missing part attached