I have a problem with my shower, is there anyone out there that i can get sme advise from?

I put in a tile shower in my bathroom. It had a tub origionally.I put !/2" durarock on the walls.Cut a trench in the slab and tied into the tub drain. I used a pvc liner kit for the pan. I used "Custom Blend packing mud from Home Depot" to make the pan,used thinset to set the tile in the pan and floor and wall mastic to set the tileon the walls. The grout joints on the wall were 1/4" .Used sanded grout to grout the shower because all of the joints were over 1/8". About a week after I used the shower for the first time I noticed some of the lower verticle joints looked like they were holding the moisture longer than the rest of the grout, not the pan joints just the walls, Now it looks like they aren't drying at all, they stay wet. It is only the first two rows that stay wet. I have put in several showers and never had this problem, even where ther were tubs. PLEASE HELP Jim