Firstly, secondly, thirtly

Hi everybody,
I'm a belgian medical student and for the moment I'm working hard on my thesis. Here's my question: In the following text I want to explain three factors that are contributing to 'something' (the content is not important). The way it's now written feels very unfluently to me. Does enyone has an opinion on how to make it more fluently


"This difference could not be explained by the difference in immunosupression so other factors have been suggested. Firstly there is a high rate of early technical failure primarly because of vascular thrombosis of the pancreas graft. Secondly it appeared difficult to detect rejection epidosed early enough to revers them. Thirtly there were frequent technical complications related to the management of exocrine secretion of the organ."

thank you to take the effort to help me
I appreciate it al lot
Ilse