Other than wound leak (which I assume has been ruled out), the low pressure is probably from chronic inflammation. All you can do is to treat the inflammation and give it time to rebuild. With lots of surgery and inflammation, the ciliary body (which produces the intraocular fluid) shuts down. You have to let it rest and recover before it can produce fluid again. But if the ciliary body was damaged, then the intraocular pressure may remain low.
Hope this helps and thank you.