Reasons for the failure could be caused by following two reasons.

1) The total Evernote's notes to be synced could be larger than 89MB. Check with Evernote's folder tied to be synced with Reader. By the way, Reader syncs all notes created by Evernote Clearly regardless of Note folder (well, it depends on your preference setting on the Reader's Evernote app).

2) The Reader may need to reserve some free space for writing and updating system management files invisible to users, so they must have set a minimum free space requirement. Making more free space always helps and performs better. Not just for Reader, but other products using storage devices.