Welcome to MDC, Mayamama!
First off, make sure you think she's not getting enough milk because YOU don't think she is, not because someone has scared you into thinking that. The number of wet and poopy diapers she is having should be your biggest clue. A lot of people had me worried about that when DD was a newborn and we were using shields, but she was actually getting plenty of milk. My supply has always been plentiful, and she is a fat and beautiful 3.5 month old. However, a trip to the lactation consultant would probably be helpful either way, and your supply is not something you should play around with--I'm just saying it is still possible to have adequate supply with the shields, so make sure you are looking at the signs.
Next, I've read that if you are using silicone shields, which most of them are now, you should NOT cut them off as PP suggested. From what I've read, that was good advice for the old latex shields, but can create sharp edges with the silicone ones that can cut Baby's little tongue.
Finally, just wanted to say that since my DD was getting enough milk from the shields, I decided not to stress myself out about weaning from them and just to enjoy nursing the way we did it. Every now and then I would offer her the breast without the shield, and figured if she didn't take it, we'd just still keep on loving the shield (which we affectionately nicknamed The Magic Nipple). Finally, about two weeks ago, DD decided she actually could latch without it just fine, and now we are down to about 50% nursing with the shield, and 50% without. I'm sure in another few weeks we won't be using it at all. I'm so thankful that we were able to breastfeed even though my baby wouldn't latch, and I'm thankful now that she's old enough to have finally figured it out!
Good luck mama!