Mechanicals experiended:

UA - DC-10-10, 737-300
AA - 757-200
QQ - MD-82

UA began receiving its A320 fleet back in '98, making it the newest additions along with the A319s. Perhaps its a teething problem with its maintenance and parts distribution systems in a relatively new type?

Personally I like the type on UA for its guaranteed new cabin interior instead of the guessing game each time I'm booked on one of the other types.