Collets can slip for a variety of reasons. First make sure you measure the length of the collet. Then measure the total length of your bit. Then subtract the length of the collet from the overall length of the bit and that measure would be your maximum stickout. You could have the bit a little bit further into the router but never any more stickout then your measurement. The reason for this is pushing the router bit into at least the depth of the collet gives you the best grip on the bit. Now do not go too far and push the bit in to far that it bottoms out the router shaft. The bottom of the router shaft may not be smooth and the vibration could cause the bit to loosen.
It could just be an oversized collet and/or undersized bit shaft. Measure the router bit shaft and it should be 1/4". Do not measure the flutes of the bit but the shaft that you insert up into the collett. Measure several different bits and compare the readings. If you have 3 of the same bit do use them to measure because if a mistake was made on one they may all be under/over sized. So get different types of bits and measure and compare.