Text content not appearing in outlook search results

We send weekly stock updates for our customers using a custom template and have a copy sent to ourselves to archive. These are all filed in a folder in outlook once sent. This way if a customer asks when the notification was sent about a particular item, we can search this email folder for the product code and let them know.
For some reason this doesn't seem to be working at the moment (I'm not sure how long for).
The content is on the newsletter as text but the search returns no results. E.g. on the below campaign, the searches 'Snow Globe' and 'P011302' return nothing.
Has anyone come across this before and any idea's how to resolve this so that these e-mails can be searched by ourselves and our customers?

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