There are several reasons to this occasion:
Your information may be entered incorrectly
You may have forgotten to enter the 3 digit card code
Your bank may have an older billing address on file
Your bank may have disabled internet transactions
You may be over your daily limit
If you make payment with your card online on Western Union / Money Gram websites, please call them to their toll free number to get help. Since they provide the money transfer service, their support is important.