Our inability or perhaps lack of willingness to sell players has been crippling.
I've no idea how Arsenal can sell Alex Iwobi for £34m, Chelsea can sell Mason Mount for £60m, West Ham can sell Scamacca for £25m yet we can't shift a single unwanted player for more than a loan with an option to buy for a fiver. Heck, United have even found a taker for Antony and he is so bad he has his own meme page.
I should add that there is more to it than incompetence. I'll post the link to a fascinating analysis I read a while back.
Basically the long and short of it is it isn't always beneficial to just sell a player to the highest bidder unless that bidder offers more than whatever the remaining contract value is.
This is a really important point that I've only recently begun to understand as I've delved down the Daniel rabbit hole of our finances.
So take Ndombele for example. Signed for circa £65m on a 6-year contract with wages of circa £10m per year.
This means for example in year 3 of his contract, it was worth 65 - (11.5x3) = £30.5m.
So to show a profit for PSR, we'd need someone to bid higher than that amount. That was never going to happen.
Even at the end of year 5, we'd need an offer higher than £11.5m to come in, in order to show a profit.
In the event that those don't come in, it becomes better to send him out on loan and at least save on a chunk of his wages, rather than sell him for less than the value of his amortized contract, which shows up on the books as a loss.
I've done a terrible job at explaining that but take a look here for more:
https://www.reddit.com/r/coys/comments/1eg55yt/understanding_transfer_policy_through_the_lens_of/