Imperfect pints (poor - head retention, cling, and gas breakout ) mostly come from glasses contaminated with a non-rinsing film, some say up to 90% of drink presentation problems are from this cause.
The solution to this problem is as easy as ABC
A. Identify the film problem.
B. Fix the film problem
C. Stay film free.
1. Film is so smooth that liquid cannot get a grip on the glass inner surface as it were, it breaks and forms drops. In such a glass stout, beer, lager will not adhere to the inside of the glass, and the head will not cling. With a filmed lager glass the etch in the base will not be sharp and break out the gas and so the head will not be fully fed and the drink will not look as lively as it could.
2. Depending on the glass film may be slight or cover the entire inside area, it may be only on the inside lip area as a result perhaps of detergent/rinse aid draining and drying in this area. Whatever the extent of the film on any particular glass, make a judgement about the overall amount of film on a sample (10 glasses). While zero film is to be aimed for, in practical circumstances around 20% total sample film indicates remedial action or Renovation is a good idea.
3. Film can arise from a variety of sources. New glasses can carry a mould release film. Normally film arises as a result of contamination from (a) detergent/rinse aid overdose/underdose or incorrect chemical selection, (b) glasswasher wash/rinse problems, (c) excess waste protein in wash water.