I recently did a retrospective miles booster but it took over three weeks for it to work.
Each time I tried the call centre had a message that my card had been declined, but it hadn't and I ended up with over eight pending charges on my Amex. It took a number of calls before Points.Com accepted the issue and finally the booster worked. I was fortunate that I managed to find one individual at the Crawley call centre who accepted there was a problem and took responsibility to follow through with their IT support and Points.Com, and of course the booster bonus came in whilst I was waiting
Spacedog I believe has had the same problem in the past. Once the payment went through it took the stated three days for the miles to hit my account. My reward flights were held for me during the delay as Virgin accepted there was a problem at their end - is a three week two day hold a record?????