Ongoing flaws in midfield, for one. The closest we've had to one in recent memory was when Matic was in his 1st season and we also had Herrera in there.
Cas and Eriksen were temporary solutions, rather than a transformation of our midfield set-up. Even at the time we were getting over-run too often, and now both seem physically spent or at least even slower, and Mount doesn't look like a number 8 that can deal with defending And creating things in an essentially 2 man midfield (given how 'free' Bruno plays as the other attacking 8). There are financial constraints, sure, but there are also lots of midfielders who could make us more balanced and were available for less than Mount. Ideally, we should be signing two, partly by getting rid of players who can't deal consistently with top-level technical and positioning-orientated game, like McT, even if we're holding out hope for Mainoo, and then maybe Gore next season.
Look at how clubs lke City recruit in depth around profiles; PSG have focused a lot more on midfield in last 2 years and brought in a spread of options, and look more balanced (it's more forward indiscipline that has cost them in CL). Real Madrid ensure that they consolidate midfield options and have a balance. Even Brighton now are grounded in having at least 5 first team 'options' for 3 slots that can all fit in even as they do slightly different things.