From April 2017 all CIS subcontractor verifications must be done online. Some agents are not aware that they need an extra level of authorisation in order to do this. If you are not authorised then the Government Transaction Engine will reject the CIS verification attempt (even if you able to successfully file CIS300 monthly returns). It is essential that all agents should organise their authorisation before attempting to file CIS verification requests online.
More information can be found on the PAYE for agents section of HMRC website.