Some of your plants do well with less light than Dendrobiums need. Phals &
Paphs are lovers of lower light. Oncidiums need more, as you found when the
addition of more light gave you spikes. Dends need considerable light in
order to bloom, and SOME Dends need a winter "rest", i.e. no water or
fertilizer from late November until new growth starts in late winter or so.
Please don't ask which Dends need the rest, but I can tell you this much:
Nobile types need it, and Den. discolor needs it. I grow a load of Dends,
and most don't get that rest and bloom all year.

You don't say if the Dends ever bloomed (when you bought them, were they
blooming?). If not, it's always possible that they are too young to flower,
but it doesn't sound like it. Try to get on a regular fertilization
schedule, and most important of all: light, light, light. Good luck!

Diana