It could really only be 3 things, Likes been said sounds like the houseing need focusing in by twisting it, Or try moving the bulb afew mm in and out, And if that anit it id say its a fault bulb with to much glass nipple bit at the top of the bulb,
Get a new bulb and see, Or if your mates have a lightforce try there bulb in your lamp to see if your still having the same problem....