ATM Error

My mother sent herself instant money, when she tried to withdraw, she only recieved part of it and not the full amount. She logged the query with a branch and was told to wait for an investigation. The initial resolution was that they had not found anything. She told them that she didn't recieve her full amount due and they logged a second investigation which came back and said that there was an error from the ATM. They said they'll send her another voucher with 3 days and they haven't. This incident happened in December. She is yet to still get another voucher.