First of all, what do you mean by "caught". If you mean it catches an edge and you fall then like someone earlier said, you need to look at edge bevelling. Sharp edges tend to "grab" on boxes. If you mean caught as in the spin is irregularish, then maybe the surface of the box is not uniformly smooth which results in you spinning nicely then you hit a non-slippery patch and the sudden slow down throws you off. If you mean "caught" because after you 270 on you are looking backwards and cant continue to spin around then you need to look around and bring your shoulders around. Continuing a spin on a box/rail is about looking around and using your arms and shoulders to keep going. If you freeze up and get your shoulders all square and fixate your gaze your spin will slow down or stop altogether. In short if you use a frontside spin onto a box (assuming its a ride on feature)you will land facing backwards and you need to turn your head and shoulders around to look at the "out" ...
Mr Wolf's tips are worth bearing in mind. For 1st time spinners the moment they lose sight of the "out" there is a moment of panic and it leads to sloppy form and edging is instinctive. Your shoulders and hips need to be parrallel to the feature as much as possible to avoid tipping so if its a down feature, you have to learn to adjust accordingly.
Lastly alot of spins on and onto features are a matter ofhow much momemtum you carry with you. You cant generate much spin once you are on a feature so you need to dial in your speed and how much "oomph" you put into your spin. If you go on wide box and want to spin 3 complete rotations before the out then you got to wind up. Also most spinners dont ride on straight, they kind of carve onto the feature and let the circular momentum from the sidecut drive their spin (kinda like set-up turns on a kicker, but remember to pop onto the feature, dont carry the edge from the carve onto a jib or u'll fall).
Hope this helps.