Depends!
A gen purpose RCD to BS4293 should trip in less than 200ms at 100% of the rated current (in your case 30mA).
An RCD to BS EN 61008 in less than 300mS. If it is being used for supplementary protection (ie for sockets providing outdoor equipment) it shouyld open in less than 40ms at 5 times rated current, ie 150mA.
So it could be working perfectly well, but, there again, it might not be. You've given too little info for a cut and dried yes or no to be given.
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