I'm sure you've tried all the other things like switching injection sites etc. What finally did it for me was the following
Step 1: Move your shot to every 6 days, do until you stall again.
Step 2: Move your shot to every 5 days, do until you stall again.
Step 3: Slowly start adding in Reta and back off on the Tirz, this should get you to end goal.
Step 4: Profit?!
I'm a 2 year Tirz user who was stalled for months on 15mg on a 5 day schedule. I'm just now up to 4mg Reta and down to 11mg Tirz after 8 months. I started at .5 reta, so super conservative as I was concerned with increased heart rate due to previous issues with heart palpitations. Doing it this way I didn't have any side affects from Reta and the scale started moving again.