Apparently the STENs problems were really due to the design of the magazine lips, or more correctly to manufacturing tolerances and user damage to them. Due to the squeeze feed from a double stack to a single feed position, if the mag was manufactured wrongly, or took a knock in the field, you had feed problems.
Better QA on the manufacturers helped late-war production STEN reliability, but you could not avoid the end users damaging the mag lips, these were always a weak point (but this would also apply to the German mags that the STEN mag was copied from too).
Vertical feed (Owen) helped a lot as there was a gravity assist as well as the spring I suppose. But the ultimate fix was to allow a 2 position magazine feed with the Sterling, but that was post-war.
I quite liked the Sterling, if I was platoon radio operator say (light! small! - no blank adatptor, so no need to
clean the &*$er! at ENDEX
. But these SMG had a problem that if you whacked the bolt back (even if on safety), you could get a runaway discharge. Typically, debussing off a lorry, if you whacked the butt on the pavement, forex. Therefore you were never supposed to carry a loaded Sterling while on a motor, at least in peacetime. (In combat, you would be allowed leeway, but just be careful when you debuss from say a lorry, where there was a big drop from the deck to the ground, or you were doing a combat dismount (some vaulting over the lorry's sides as well as some via the tailgate))
Cheers
Andy