A strange issue!

The company I work for has a corporate rate with Hilton.
Normally when I book Hilton Group Hotels I use the iOS app on my phone, mainly because it's quicker, but also if I need to cancel or change it's far easier than logging into the corporate travel agents own website.

Whenever I check rates for a HGI or Hampton using the iOS app, it never displays the corporate rate.
I had assumed it wasn't valid at those brands, but today after going through the corporate travel booking site i see the rate available at both these brands.

I have checked on the Hilton.com website and same thing happens, anybody know why or how to resolve?
I have checked my HH account and my corporate rate is in there, it just doesn't seem to be applied unless I use the corporate travel agency site.

I just made a booking using the corporate travel agent site and then contacted HH customer services, who confirmed that my booking had picked up the same corporate rate code I have.

Puzzled!