proving right to work in uk

Hi,

I`m new to this forum and just wonder if anyone knows if employer can refuse COA as proof of entitlement to work in UK.
I have sent EEA4 form and received COA which clearly states that i can work, yet the employer does not accept this as "good enough"
i had job offer in writing but last minute they decided to withdraw offer because they can only accept "original visa" which can take up to 6 months to process. can the employers simply ignore COA as proof?