Did you check if your cover image is identified as the cover image in the metadata and manifest?
Looking at the .opf file, you should find in the metadata section and the manifest section, entries for the cover image. In the sample I copied/pasted, the cover metadata was the last item in the metadata block and the first item in the manifest section.
Code:
<meta name="cover" content="cover_jpeg" />
</metadata>
<manifest>
<item id="cover_jpeg" href="Images/cover.jpeg" media-type="image/jpeg" properties="cover-image"/>
Otherwise, I would say that Google Play Books on Android is the issue. Have you tried another Android epub reader?