Each time you install the column in the injection port or detector interface, you cut off a small piece. These reductions in column length will reduce retention times (especially on shorter columns).
Check the initial oven temperature. Make sure it has not been increased. Increasing the initial oven temperature decreases retention times for all compounds.
Check the number of injections the column has processed. There is no firm rule about how long a column will last but columns gradually loose small amounts of their liquid phase. This reduces retention times.