In The History of Western Philosophy, Bertrand Russell writes of Liebniz's theory "as to why some things exist and others do not."
According to this view, everything that does not exist struggles to exist, but not all possible can exist, because they are not all 'compossible'.
Firstly "as to why some things exist and others do not." To talk of 'others', as in other things, is to talk of things which exist. That is what a thing is - something which exists. It is a senseless question to ask why do things which do not exist not exist. The framing of the question is necessarily to talk of these non-existents as if 'they' do exist. There is no they to which you can refer. Some thing which doesn't exist is meaningless language; some thing being a thing which must exist.
And as for Liebniz's response to the question, being that everything that does not exist struggles to exist: he is continuing to talk of non-existence as if it were existence. For something to struggle - to struggle to exist, or struggle to climb a mountain or whatever - is for it to exist in the first place so as to be able to struggle. Only things which do exist are capable of struggle or any kind of activity.
Whatever 'compossible' means is irrelevant since everything that has gone before its appearance is nonsense, and so the 'conclusion' to, or inference of, a stream of nonsense can only be more nonsense.