Hello all.
Generally speaking there are numerous causes for low quality beer from the taps. I will cover just a few to see if we can clear the issues up.
Beer that has a light egg / rotten smell can be an indicator of contamination caused by contact with brass fittings in the beer system. Stainless Steel beer systems avoid this.
Beer that seems more bitter / bitey / unusual taste is generally the result of dirty beer lines. Bracton / Lion Nathan / Fosters Group Specifications requires the beer lines to be cleaned once a week with a two part cleaner (generally Bracton DP1 & DP2 or Bracton Low pH) for a minimum of two hours.
Beer that does not retain its head can be caused by many factors including: temperature of the beer, beer hygiene (or lack thereof) , dirty beer glasses, tap bore sizes and beer type to name a few.
I hope this helps, I can help if there are more specifics provided.
Regards,
Russell