Rated 5 out of 5 stars

All credits goes to "beezilla" Thx to this advice i fixed this pesky problem.
Original text:
I guess the problem isn't the Gmail Manager but the Gmail itself. I had this same problem for the last several days (ver 0.6.4.1.6). One of my accounts didn't show snippets in my inbox and my Gmail Manager. I got my snippets back when I changed the Inbox type from "Default" to "Unread first" (https://mail.google.com/mail/#settings/inbox). Funny but this account doesn't like default type anymore.. and If I'm not mistaken, this issue started after Google introduced a new “Tabs” feature that automatically sorts incoming mail into configurable tab categories. A new function is inserted into default type.

Also, make sure you have the snippets enabled.

Gmail general settings:
https://mail.google.com/mail/#settings (scroll down) >> Snippets: Show snippets - Show snippets of the message (like Google web search!)

Gmail Manager:
Gmail Manager Options >> Accounts >> Modify >> Tooltip >> Show new mail snippets from the Inbox