Recurring transactions not recurring

Hi all

I have always noticed but never thought to ask here...

I have some monthly automatic payments set up as recurring transactions. Is there any reason why randomly some will not recur? I process my monthly bank statement and usually i have to manually create the payment (using the RT window) and always go back in to make sure the next payment date is correct in the schedule. It is not that same transactions every time. 

Thanks

Carolyn