Never ending loan terms 12 mnths become 18mnths

I have a complain about woolworths personal loan that is always extending without my concern. what i mean is that i have always requested money from my loan account with every loan taken i have always been asked for how long will I be paying my installement. but to my suprise the requested loan term will pass and i will still be paying even out of the loan term agreement. try to contact them serveral time with no clear explanation of why i will still be paying longer than term agreement. can you please assist in finding out what is the problem regarding my loan account.