I installed my new TV Box today and tried to watch C4 catch-up using the App, but I could not see Grand Designs Australia. I can however view Grand Designs Australia using C4 catch-up website. Is there a difference between the BT TV Apps installed on the box those on the actual websites for BBC, ITV, C4 for example. I was hoping I will be able to view the same content at the same time, unless there is a time delay, but will be strange if that is the case.
I am new to BT TV/NOW TV package.
It's up to the content owners and channel operators to make their catch-up/on demand content available across all platforms. Sometimes there may a rights issue that prohibits it going on certain forms of catch-up services, other times it could just be sloppyness in their part. Either way, BT don't control this content.
looking at your example I suspect it is a Channel 4 issue.
Reasoning here is that if you use the general Searth in Youview for Grand Dersigns Australia you get the available ON Demand content which clearly shows Series 8 Episide 9 (currently available for 7 more days) and Series 8 episode 10 (currently available for 14 more days)and gives you the option to watch on demand. Selecting either episode launches the All4 player but each then reports an All 4 (All4-001 error) "Well this is embarrassing Something's not working, please try again in a few minutes."
As you have found if you launch the All 4 app itself you van find no trace of Grand Designs Australia when using the apps own search.
As the content is accessible on my large screen Samsung TV All4 app it is unlikely to be a rights issue but rather a simple Channel 4 All 4 error. Also as Channel 4 have indicated to Youview (evidence the search results returned) that the content is available the error is in my opinion firmly with Channel 4.