Thats one of the problems with inertia systems - they need a minimum force to activate. Its the initial 'thump' that stars the cycle. Unlike gas systems which can be regulated to shoot anything.
I had the same problem with both my Sport 2 and Córdoba. Benelli's are notoriously finicky In this regard. The simple solution is to shorten the recoil spring approx 1 1/2 inches which should allow all 24 grain loads to cycle. It is a bit Heath Robinson but does work well .The alternative is too order a Wolff reduced power recoil spring from Brownells U.K. (£10 ish) which will cycle in my experience Gamebore Evo & Blue Diamond 24 loads but has still too much resistance for Express or Dan Arms to cycle 100%. Hope this helps