"Non-drowsy" on an allergy medication label almost always refers to second-generation antihistamines (loratadine, cetirizine, fexofenadine), which cross into the brain far less than older antihistamines like diphenhydramine.
It’s a population-level claim, not a guarantee for every individual. A meaningful minority of people still report some drowsiness on cetirizine in particular, even though it’s labeled non-drowsy.
Combination products (labeled "allergy + sinus" or with a "D" suffix, like a decongestant combo) can also affect alertness differently — the decongestant component can be stimulating for some people and disruptive to sleep if taken late in the day.
If a "non-drowsy" product is making you sleepy, it doesn’t mean you’re doing something wrong — it just means your body responds differently, and it’s reasonable to try a different antihistamine in the same class or talk to a pharmacist about alternatives.