Networking problems

Please Help

This problems happen after I updated to system 10.10.5

3 iMac on the network, a psd file is stored at computer A, I open the file on computer B through the network, made editing and I can't save the file "could not save...write access was not granted". The Read & Write is selected at the finder "get info"

After that, I can not eject the computer A from the computer B, In Have to off the power for Computer B and re-start

Can some expert help me on this!

Thank you