I think Happy hours increased drop rate messes up 1/2 and 7/8 drop rates (not exactly sure why, but it does that). The happy hour did not make it 21/8 (logically it would, but it doesn't). I think how the drop rate is actually increased glitches the number of really common drops. The server generates a number and looks it up on a table. Now the drop rate increase divides the said number, in this case by 3) (and yes, you want the roll to be as low as possible). This is what determines if you get the rare of not:
"rare_roll < rare_rate"
So if the random number is less than the rare rate you get your rare. It's probably dividing too low or something which is messing it up for the extremely common ones.
All other drops are fine (most in the game). If you want a 1/2 and 7/8 drop, don't try on happy hours, your bound to get it easily. As long as the drop rate doesn't equal 1/1 or greater you should be fine (basically 1/3 drop rates or higher).