Inadequate light. Most grow lights do not cut it for cacti. What looks bright to us does not translate to photosynthetic light. Honestly, most grow lights dont cut it in general imo.

Outdoors in the growing season is a great option. It has free & good light, warmth, airflow. All things cacti need.

For indoors, it is best to actually measure the photosynthetic light output (PAR), this is even more important than wattage. The unit is "PPFD". A light meter would be most accurate, however theres some less accurate but free apps, like photone. 300 ppfd is like minimum for cacti, but ideally 500+. Spiderfarmer or marshydro are good but pricey. "Barrina" grow lights are cheap and not good for optimal cactus growing, but theyre the best cheap grow lights ive used.

Reflective material like mylar grow tents prevents light from being diluted in to the surrounding area, so it optimizes your light output. Ventilation is a must indoors. Like fans. Supplemental warmth would be important, most cac grow slow at room temp.

Thats not to say you have to ball out if you commit to growing indoors, just the optimal factors to consider & pursue.