Bob
I have used ammonium dichromate solution as old as yours with no diminution
in its effectiveness.
Apart from such obvious possible explanations as whether the tubes were of
the same wattage or whether the ammonium at home was ammonium and not
potassium, it is possible that the glass you are using at home is blocking
the UV.
One other factor might be an increase in the effectiveness of the
flourescing materials. I have noticed that the flourescent bulbs that fit
into standard ES or bayonet fittings give out far more light than when
they first came on to the market.
Terry King