First of all, you need to be more specific (also ideally you'd use a better title of your post than "problem with survey", most that post here have some kind of problem with a survey...
).
The problem description is very vague. I don't see a way to help with concrete advice.
What does "have problem with finishing" and "survey tuns off itself" mean? Do they get an error message? What does it say?
Where does this happen? At the end of the survey? In the middle? Any details? The responses up to the point where the survey "turns off itself" (whatever that means) should be stored in the database, correct?
How do your respondents access the survey? Is the survey token based? Is token based answer persistence switched on? Because with this, people can just open the survey again and continue from where they have left off, when the survey "turned itself off".
I assume you wanted to say that your survey "is quite big".
There is a good chance that you run into session problems, etc.
Also, where is your survey hosted? What version do you use?