Credits are not substracted correctly

Hi we have experienced some problems with credits purchases. These are the 3 escenarios that have been presented.

1. When a client purchases an order with the sum of their total credits, all credits are substracted from their accounts (which is correct)

2. One user purchased an order with half of her credits (lets say 100 credits) but the plugin substracted only 40 credits instead of 100.(we had to manually corrected)

3. Other client purchased an order with some of her credits (lets say 200) and non of them where substarcted from her account. We had to do it manually

Why does the credits sometimes are subtracted correct, sometimes incorrectly and some times thy aren’t even substracted? how can we fix it ?