Rarity's levitation spell was never very strong. She couldn't lift anything more than a full grown Pegasus mare, both due to things like weight, gravity, and magical resistance. Another thing to consider about levitation is range. A Unicorn's spell is only effective within a certain area, and the only way to increase that area is with practice, training, and a massive pool of magical energy. The further away something is, the harder it's going to be to keep casting on it. It's best to think of it as sound waves, a Unicorn's horn is the instrument, while the magic they use is the music. Eventually the sound of music will fade too much for the spell to function. And the only answer is to have stronger lungs, to blow more music through their horns. A Unicorn can cast more loudly, but that will usually muddy the spell. And Rarity was trying her best to be as quiet as possible, which reduced her effective range even further.  