most probably the headlight adjusters inside is broken as is made out of really brittle plastic. for the non angel eyes headlights it would be a easy fix as the headlights can be easily disassembled. parts can be ordered through BA as its a oem fix. part number is 63 12 0 027 924.
however, if yours is the CELIS "angel eye" version, be it OEM or hella version, its gonna be a permanently seal unit and to open it would required having the lights bake in a oven to soften the seal. NO replacement parts available from BA, BUT there is still hope as there are 2 companies with parts specially made for us. check out odometergears.com and ca-automotive.co.uk for the parts.
the ones from odometergears at 85usd per side is made out of plastic, it looks the same as the original part in the lights but made out of 20% glass filled delrin. the ones from ca-automotive at 39quid per pair headlights is machined out of aluminum, very nicely done and quality superb. i have the image of the parts attached below.
this should be a permanent fix to the problem, it will be slightly pricey but still cheaper than getting a new set of lights.
i have just ordered a set from CA-automotive, if its not too hard to replicate it at the machine shop, i will let you guys know.