Purchase Ledger Invoice Entry

Can anyone please help me with a problem on our purchase ledger..

When I add a line to an invoice, I don't get the correct g/l account as
specified against the p/l account (and supplier). I'm also unable to
modify the g/l distribution because the add / update / allocation /
delete buttons are disabled.

I'm guessing I've missed something simple or stupid (or both) , any
ideas anyone?

Nigel Roberts
MC Sheet Metal Limited
Wales, UK