ITO is no more.  It was rebranded PTO, I think two years back.

Anyway, two possibilities come to mind.  You may want to verify your records and/or inquire with your clients just to rule these out.

1. Your client used a bookmark to the old (pre-TY2017) Link portal to access their account.  The portal for TY2017 and later is prolink.intuit.com; or
2. Your client used a previous email address to open their Intuit account but you have since updated that to a new one using Edit Client within the PTO dashboard.  The invite and requests will be sent to the new email address but all your requests will still be populated only under the account with the old email address - this was reported to Intuit earlier this week but I have yet to receive an update from them.