Most people forget to set the system to recirculate and even close the windows when using the bombs and you must leave the system running for a good 15-20 mins after the bomb has expelled, most important don't sit in the car when you set the bomb off.
Have used bombs many times and always with good results,
Modern systems and ATC's tend not to suffer this problem as they run all year and are used in the winter as well to defog the inside of windscreen, but manual systems can sit static for months at a time not being used and has been said its the matrix/evaporator where most of the problems occur. Try switching on your manual AC a couple of times during the colder months and this should help with the problem and keep the AC systems seals lubricated so you loose less gas during the colder months
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I would be so fast if I could only get going 
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