Sylvite, a potassium-rich evaporite mineral, might be expected to reset well, but it gives "unacceptably young ages", and is therefore not used. Other minerals, such as muscovite and biotite, sometimes apparently fail to lose all their argon and/or actually absorb argon, since they tend to give "too old" ages (even more than the "age of the earth"!).
Some rocks that have been heated after formation show younger K-Ar ages than similar rocks in the same deposit that have not been heated. Scientists who believe the evolutionary timescale explain the multiple "too young" dates by attributing them to heating, metamorphosis, etc.
Many rocks which show no signs of heating or metamorphosis yet date "too young" are also considered to have lost argon, even though experiments find negligible argon loss from minerals at normal temperatures. Many "too young" dates are therefore unexplained from the standpoint of the evolutionary timescale.
2010 Arthur V. Chadwick, Ph.D.