EDIT: I misread the OP and thought he was in a wet suit, so much of what follows does not apply. Sorry!
You don't provide a lot of information, especially about the thickness of the wet suit, but I am guessing it is a thick one for my response. If you are wearing a thick wet suit during a safety stop, several factors are combining to make your life difficult.
First of all, understand that it is much more difficult to maintain buoyancy during a safety stop than at depth, no matter what you are wearing. The closer you are to the surface, the more the gas spaces in the BCD and wet suit will expand and contract as you rise and sink. At sea level, going from 33 feet to the surface, those spaces will double in size. In the typical range of depths in which divers usually hold a safety stop, those spaces will vary by about 25%. That same range of depth change at about 60 feet will have only about a 10% change in volume.
Next, a thick wet suit makes things more difficult because there is much more expansion and contraction of gases to contend with. The degree to which a thick wet suit expands and contracts with changes of depth can be significant.
A further complication is that a thick wet suit requires you to be overweighted during a dive. That is because you need the weight to descend near the surface, but you don't need as much weight once the wet suit contracts at depth. That means you will have more lead at depth than you need, meaning you must add more air to the BCD to balance it. Even at only 15 feet, that extra amount of air adds to the difficulty of remaining neutral.
Finally, the majority of divers are overweighted regardless of their actual needs, and in my experience, that includes especially divers with thick wet suits. The more anyone is overweighted, the more difficult it is to control buoyancy, and as described above, that problem is amplified at shallow depths. Every extra pound of lead demands about 15 fluid ounces of air to compensate. I have no idea from what you wrote if you are overweighted, but it is possible. I did OW dives in fresh water with a student who had done his pool work with another instructor, and he had done it in a 7mm wet suit so he wold know what his weighting needs were when he did the OW dives. He told me he was certain he needed 22 pounds, and I made him start with about 18. It was way too much, and he could not control his buoyancy on the first dive. When I finally got him down to 10 pounds, he found controlling his buoyancy to be no problem at all.
Ultimately, if you are weighted as best you can be, it all comes down to getting more and more practice.