I have one of the "not recommended" ones and am wondering now what to do. Do you have one?
Some booster seats might not be protecting your child as much as they should.
A review found 13 booster seats did not put children in the best position to be protected in a crash. The report says those seats might increase restraint use by making children more comfortable, but they don’t position belts for optimal protection.
The study was done by the Insurance Institute for Highway Safety and the University of Michigan Transportation Research Institute.
Here is a list of the seats both groups recommend as Best Bets:
Combi Kobuk
Fisher-Price Safe Voyage (with plastic clip)
Graco TurboBooster
Britax Monarch
Britax Parkway
Fisher-Price Safe Voyage (highback)
LaRoche Bros. Teddy Bear
Recaro Young Style
Volvo booster cushion and Safeguard Go; when used as a backless booster
Here is the list the groups recommend as Good Bets:
Highbacks Combi Kobuk
Graco TurboBooster and Safety Angel Ride Ryte
Recaro Young Sport and Safety 1st/Dorel Apex 65, when used as highbacks
Here is the list the groups do not recommend you buy:
Compass B505
Compass B510
Cosco/Dorel Traveler
Evenflo Big Kid Confidence
Safety Angel Ride Ryte
Cosco/Dorel Alpha Omega
Cosco/Dorel (Eddie Bauer) Summit
Cosco Highback Booster
Dorel/Safety 1st (Eddie Bauer) Prospect
Evenflo Chase Comfort Touch
Evenflo Generations
Graco CarGo Zephyr,
and Safety 1st/Dorel Intera
However, experts say parents should not interpret the evaluations to mean that poorly rated seats are not effective. So, what's a parent to do? I'll make some calls on this today and tell you what I learn.

This is the one I have. I didn't remember the name of which one I'd bought, but I definitely know what it looks like. Only after doing a google image search did I realize I have one of the 'not recommended' ones. Bought it at Babies R Us. Matter of fact, we have two, since Grandma needed a seat, too. I hyperlinked google image searches of each of the seats on the 'not recommended' list to make it easy for you.
BTW, the federal government recommends car seats for children up to 40 pounds and booster seats for children over 40 pounds until they are eight years old, or four feet, nine inches tall.