How do I retain drummers in my band?

This is an issue that plagues every band at some point.  It is generally felt worse in the smaller bands.

I don't want to get into one-off political issues or personality conflict situations, but to address the larger issue of why would a drummer leave one band to join another.

If you think about it for a while, the answers begin to come.  A drummer can be trained/grown in about a year.  A piper takes at least three.   Consequently a capable drummer will usually outstrip the capabilities of the band (s)he is trained in.  A motivated drummer is ready to move up to another level of play after a couple years.  If there's sufficient camaraderie or dedication to the program, (s)he may stick around even if the musical challenge isn't there.  (I owe a large debt here and I know it!)

So now the answer is obvious - to retain your drum corps, challenge them.   Provide opportunities for a "Drum Salute" in performances.  Provide challenging music and ask them to play non-standard, but written scores.  Take their input on medleys and tunes.  Acknowledge them publicly.  Encourage them to play with you at solo/private events. 

The ultimate answer is to challenge your drum corps to take the spotlight as often as possible.  Given opportunities to shine, you'll find that they will figure out how to build themselves to be the best pipe corps that they can be - just as your pipe corps should be doing.   If you do this, you'll find that drummers will want to be a part of your band and they will come to you!