Yeah, I would go with Dan's suggestion. What are you doing for compounds? Some bentover rows, chinups, etc should help build up more strength in your arms.
Alternatively, I would consider not adding in curls until the 5s. Then go 15 15 20 20 25 25 or something along those lines. You could also use smaller increments to try to gain more strength, assuming you have small enough plates. Maybe 15 17.5 20 22.5 25.