It sounds like the inner part of the mag spring has broken off, allowing the spring to only partially wind until the inner slips around and unwinds the spring in one quick burst. If the mag is new, send it back to the retailer to sort and in the meantime, try to borrow one from someone to confirm that there is nothing wrong with the gun itself. Alternatively, if you feel like repairing it yourself, hicap mags are fairly simple to strip and a new spring is quite a cheap fix.