After over 300 rounds through my Shield 9 I still could not lock it back without using a bench to push it against. Finally I purchased some GT-5000 grip tape (same as the Talon Grips material). Amazon has it for $6.99 here.
After making a pattern with masking tape I cut it to size and stuck it on with no problems. It completely solved my problem. Now I can lock it open with no problem.