Cheque payment not available to allocate to an invoice

maria

Hello,
With proforma invoices, i 'write a cheque' when i pay them then once the goods and invoice have arrived, i allocate the cheque to the correct invoice. 
But i have one Import transaction when the cheque was created a couple of months ago and the goods and invoice have just arrived.  The invoice has been entered but i can not allocate the cheque payment to this particular invoice.  
I did another payment to the same supplier yesterday and this payment is showing up but not the previous one.
Both payments and the invoice are showing as outstanding for this supplier
can anyone help??
thanks,