The usual reason is lack of light. Many new growers take the term ‘houseplant’ a bit too literally and presume that the phal will need about the same light that a foliage houseplant needs…and then despair at the lack of flowers. Fact is, if the phal is given too little light, it will vegetate very nicely and produce pretty, dark-green leaves…and nothing else.
Asking the new grower to guess at what 10% of full sunlight looks like is a bit much, but a reasonably close estimate can save you a whole growing season that may be needed to show that a given spot has not enough light for the plant to flower. One thing I really fret about is that some new growers will give up during that first season and try something else.
So, the guess at what is enough light to make a phal flower is an important guess. If you have to guess on the wrong side of 10% of full sunlight — which is what the phals need to bloom — guess on the high side.
Let me qualify that comment: ‘A little on the high side’.
A little high gets you –
yellow-green leaves and small flowers
A lot high get you –
a sunburned or even a dead plant
A little on the low side gets you –
sparse, but large flowers
A lot on the low side gets you –
no flowers, but a pretty, green plant
The phals will tolerate considerable overdosage of light before getting into any trouble. I’d rather see new growers overdose them a little than underdose them, because at least they see the flowers.
I have never known a new grower to overdose a phal on light except by leaving them in direct sunlight and heat in a parked car with the windows rolled up. When that happens, ZAP! The plant is dead.
There are a few other reasons why a phalaenopsis orchid will not bloom given the other recommended cultural conditions, but they are infrequent. Flower spikes may sometime fold and collapse when splashed with cold water in winter. Immaturity or poor health of the plant, of course, will usually deter flowering. Sometimes a plant will flower while it is in bad health as an act of desperation…to keep the species alive. It’s a pathetic sight; the poor thing trying to carry on by flowering even when the flowering may rob it is strength it needs to stay alive. I’ts a jungle out there.
Don’t let it happen to your plants. If a sick plant throws a flower spike, cut it off at the base and nurse the plant back to health before allowing it to flower again.