When you use a beard trimmer on a regular basis, remember to check the length setting EVERY time you use it.  You never know when the woman of the house has borrowed it to trim eyebrows or private areas or who knows where.  In all likelihood they've changed the setting down to one of the shortest available, and it is YOUR responsibility to set it back for yourself before the next time you use it.  Otherwise you are bound to start regrowing the beard from Stage One.