There are two ways the spin will collapse, the most common is from pointing too high and you just can't trim it in enough. That's the easy one to correct. The other one, and one you may have been experiencing if it's your first cat ride with the spin, is when you are screaming along on a reach, and get hit with a gust, the skipper immediately heads down (or had better, lest you swim) and the spin collapses, because now you are "over trimmed" for the new lower angle.
As the skipper quickly bears off, the trimmer has to ease the spin to keep it going, but alot of times, when a quick gust hits, you may want the spin to collapse or you will stuff it and swim. In that case, once under contol, you ease the spin sheet considerably, get it drawing, then trim it for what ever angle the skipper wants to sail.
You can also collapse it by going too fast, as when you are surfing down a wave, going deep downwind, no amount of trimming or easing will re-fill it untill you slow down a little! But when it re-fills, be sure the mainsheet is tight or the mast may snap forward.