Thermoplastic nylon used in shuttlecocks is hydroscopic and the brittleness is affected significantly by changes in air temperature and, in particular, relative humidity (RH). There will have been a very large difference in durability of any kind of shuttlecock, including feathers, from December to January. December is often wet and this year broke all records, with RH values of above 60% at room temperature consistently. Recently with an anticyclonic weather system prevailing, as is common in January, the RH has been about 35%. At this level of minimal saturation, any kind of shuttle will last less than half the time than it would in average conditions. The only way to be sure of comparative durability of different products is to store them together and trial them in parallel.
However, a suggestion to improve was to steam or mist shuttles, but this can affect them in a number of ways. It could affect the crystalline structure of the plastic making it more brittle or less resilient which affects the feel and also can affect the glue. There is little one can do as the very thin sections which are vulnerable absorb or lose moisture to the atmosphere very quickly. Keep them in the tube with the lid on as long as possible; they leave the factory in ideal condition. Try not to store shuttles where they will see extremes of temperature or humidity, high or low.
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