Attended transfer issues

Hi I have an issue with freepbx and both a snom 320 and 720 phone when I try to do an attended transfer

a customer rings and I answer, they want to speak to staff1
I put customer on hold and ring staff1.
I then press transfer and tick to transfer customer, this works ok till we get a second customer ring whilst in the transfer process, If A SECOND customer rings in it transfers them to staff1 not the person on hold.
I have also tried pressing transfer then line1 then tick but it still connects line to to staff1
Is there something I need to configure in freepbx to stop this happening