My supervisors take long time to respond to me I am a PhD student. I need to finish my study in 6 months.

My story and problem:

I always send my work to my supervisors on time. For each work (objective), they take about one to two months to respond to me. Therefore, I need to send them a reminder after one month. After my reminder, they ask me to resend it again as they forget it and do not want to search for it. Then, they take about 2 weeks to respond. 

After that, they asked me to do some more work, which I do and resend my work to them again. They agree that this version is good and ask me to do my next objective based on my study plan. 

After finishing all my tasks and sending them my thesis, they asked me to go back through all my objectives and do further work. These objectives have been already proven many times in many different areas and are not related directly to my topic. I explained to them that I will be in trouble if I do not finish my study at the time that is specified by my sponsor. The university has put me under review as I did not finish on time already. Also, I have had to request two extensions from my sponsors. 

In addition to their delay in responding, they sometimes travel overseas for about 6 months without regular contact. This happens every year. 

What do I have to do now? Any help, please?
There isn't a lot you can do on your own. The supervisors have their habits. The best advice I can give is to try to find someone to speak to them on your behalf to get them to help you meet the deadlines. You want an advocate that will be respected by the supervisors. Deans are good for this sort of thing, actually. 

But if you can meet face to face with people, either the supervisors or an advocate to help impress on them the urgency it would be better than trying to do things by email, which is asynchronous by design.