If it isn't a technical issue then it might be that you've missed an action along the way, maybe?
Here's a checklist from Tally Ho's walkthrough up to that point:
Checklist for Day Three:
Office: Take dagger and Observatory key
Observatory: Take Files and Raven from cabinet
Send Three telegraph/faxes
Operate Holograph according to instructions in Files
Receive three faxes
Office: Take Chapel Key
Chapel: Break key; find rope and grapnel; combine.
Climb on roof; solve Sirdach puzzle