MTN refuse to pass credit

I have been a customer of Nashua Mobile for 13 years and had to move across to MTN when they close down. I received a credit note of R 3500 on 1 December 2014. This was due to Nashua not being able to give me a Samsung Gear watch when I upgraded as they had no stock. With the account being passed to MTN, MTN has been stalling to pass this credit to my new account, asking for documentation, which I have sent to them electronically and in paper to their branch in Somerset Mall. I am being told they are still investigating, this is now going on for for almost 4 months!! MTN is quick to suspend you if you owe them money more than 14 days. Why must I wait so long for my money which they owe me???