A significant issue is that trenches weren't singular lines of defence, but rather multiple ones designed with defence in depth as a guiding principle which had massive ramifications.

Gaining the outermost trench wasn't all that difficult, holding it however was a nightmare. The outermost lines were lightly defended with the bulk of the troops stationed on the ones behind that, away from enemy artillery while the front line was still well protected by theirs. This meant if you gain that outer trench line, you wound be facing immediate counterassaults from large units of fresh infantry as well as being under fire by enemy artillery whilst you were still trying to reorganise your units and move your artillery up to defend your line. The enemy also held other high cards such as having direct communication trenches leading to frontline trenches, defences between the first and second lines being designed with counter offense in mind and having clear, stable lines of supply behind their remaining trenches whilst you were stuck with the question of how to lug HMGs, ammunition, wounded and a hundred and one various things through the quicksand like quagmire that was no mans land and get your artillery forward to support you (which given said forward positions were square in that quagmire was a difficult task in the least). Logistically and tactically, you were quite utterly screwed despite your success.

So even if you managed to dig a trench into the enemies outer line (and not have the daylights shelled out of you in the process), you were still massively exposed to counterattack, especially by the Germans who were notorious for quickly and aggressively pushing back, whilst lacking artillery and logistic support.