Searching email

I have had an email account since 2000, lots of email to say the least. In particular, there is one party I have exchanged a volume of emails over these 19 years - but when I search for those emails using the app on my android phone, the search only returns very old emails (early 2000s) and maybe 1-2 current emails - no matter what I do, I cannot search to approximately 10-15 years of saved emails in any way. I've submitted this issue 3 or 4 times now with no reply. It seems that maybe there's an indexing issue, however I just need access to all of the matching results for legal purposes. Thank you.