You need to give information about where you live, the conditions this plant is growing under...light, temps, indoor/outdoor, etc.

Just looking at your photo, my guess is not enough light and poor potting conditions. You are possibly overwatering and fertilizing.

Without more info from you, that's all I can guess.

Jane

I completely agree with Jane. Not enough info but probably not enough light is the main reason.

NYC area and its in the same location, and being treated the same way for it to bloom for me at least once. Same watering and fertilizing method and indoors in a bright northeast light and now with the weather getting warm, I put it outside in bright shade, no sun.

"Bright Shade" is not good. It should be in sunlight in the warmer months of the year when you have it "outside".
That sunlight should be filtered by shade-cloth or similar appropriate to the latitude where you live.
"Outside" here means a glass-house, Shade-house or appropriate spot in the garden.
Here is a pic. of a shade-house at latitude 33S. Two layers of 50% shade-cloth are needed in summer. One layer in winter.
You need to find a spot where the plant will get sunlight without sun burn. Eastern side of a tree or shadecloth?? Hopefully growers in similar climates to yours will add to this.
{{gwi:174810}}

Listen to Arthurm. Your plant needs more light. You should only fertilize when the plant is starting to make new growth which is usually spring. You can give a weak fertilizer throughout the year, but I don't.

Grew these plants for many years in NY and they went outside in summer and gradually handled full sun on an open deck. This must be done gradually to prevent burning the older leaves. The newer leaves will adjust to the higher light as they grow.

I would fertilize now and stop after the summer (or at least cut back to 1/4.)

The yellow leaves are probably old leaves which is normal for a Dend to lose. The potting mix does not look good and the pot shows signs of fertilizer salts. I would repot and put the plant outside for rain and fresh air.

Jane

Thank you all...I really didn't know that they can take the sun...will do.

Boothbay should gradually expose it to sun though, right? (I am learning as well).

Speaking of lack of light - what are the signs in the photo? Light green leaves? Should be darker green in better light, or what? Any other signs?

The leaves should be a light green. Dark green is not enough light. The mix looks bad and the plant should be repotted.

Jane

Hi - I agree on the issue of light. Dendrobiums like this "cane" type can handle full sun.
I noticed you have it in a pot. Mine is in a basket with almost no media. Perhaps you should check the roots for rot. In the summer, I hang my cane dendros outside, full sun, provide copious amounts of water and fertilizer. Let them cool off and dry out mid October for at least 2 months. When you see buds forming you can water and baby them again.
Good luck!
ML from western MA (WMass)