Problem with Main Beam

[brandom]

Hi hoping someone can help. I have a 2007 VW Passant 2.0 and the main headlights have recently appeared very dim on the road. I took it to MOT and it failed as main passenger side headlight was below level needed. The tester thought it was a loose or faulty bulb. I checked bulb and sure enough it had fallen out of housing so I pushed it back in and twisted to secure it. I took it back for retest and it's failed again as tester is saying both Driver and Passenger headlights are showing NA and not reading on their machine at all. He showed me the light on their machine and said they are very dim and yellowy in colour and should be sharp and white. I changed one of the bulbs but brightness is still missing. Any ideas what issue could be and is it a big problem. Many thanks for any advise and guidance