An invoice is initiated for approval, it says 'future' with the approver's name, but does not appear

Hi gurus,

I have a couple of invoices for a particular supplier.
They were validated and initiated for approval.
Now the invoice approval history says 'future' with the name of the approver.
And when searched for workflow notifications TO the approver or FROM the invoice approval initiator, there is no notification sent.
This is happening to invoices for a particular supplier.
How to solve this issue.

Thanks
Raj