I'm installing a shower and can't seem to get the waste pipe right. I have 2 inch PVC attached to the drain and I need to convert it to 1.5 inch to attach it to the waste line. I 1st tried it with about 6 inches of 2" PVC coming out the drain from the shower, then coverted to 1.5" P trap and ran it to the waste line about 1.5 feet of length with a 90d elbow. No leaks, looks great, but won't drain. What am I doing wrong? Do I need to use all 2" PVC and P trap until I reach the 1.5" waste line?