Noreen....Many years ago we came across the same
problem and found several problems.  Waterbath
contanimation is the most common, next was the
embedding of the positive and negative control in the
same block.  The final one that really set everyone
back was the bacteria found in the distilled water
system.  We would fill our carboys up in chemistry and
use the water over several days  and sure enough the
water was growing all by itself.  You may want to wash
everything with hot soapy water, rinse well with fresh
distilled water then dry the containers in an oven and
then make your solutions from water you buy... if you
want to try this theory and make some your old way and
see what happens.  We now have our water tested by
micro...the waterbaths are cleaned daily with hot
soapy water and rinsed with fresh distilled.  I'm not
use how they are doing the control blocks..not my
area.  Good luck.

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