Searching for Exchange emails on iPhone

I am trying to find an email from my iPhone which was received a few weeks ago. If I do the keyword search from my PC it finds it correctly. If I do the same search on my iPhone it only finds a separate email chain from March 2015 which also contains the same keyword (this also appears on the iPhone search). 

Any ideas why my iPhone search of my work email appears to only search an index built from pre-July 2015?