Moorcroft Debt collector settlement

Hi, I've been in a DMP for a few years and I don't have long to go until I have cleared all debts apart from 2 accounts that Moorcroft hold (2xVM accounts) I have called them several times to see if they can offer a reduced settlement on all occasions they have told me VM won't allow them to reduce the balance to settle the debt, I have now written to them to see if there is anything that can be done, the debt is a few years old so my DMP said they should budge but they are not, there are normally 2 ways an account gets passed on 1 the collector buys the debt for very cheap and the reduced settlement favours all parties and the other is that it gets consigned to them and there will be a target of 50% to collect for example if I 1k and the target is 50% they will be happy with £500 they get £250 and VM get the other halve this is what my DMP have told me but why wont they budge on a 2/3-year-old debt?