What are the symptoms you’re seeing that indicate the cactus is having trouble growing? Is it not growing at all? Do you see rotten spots at the base of the plant? Is the new growth skinnier than the old growth? I’ll assume you’re referring to the spiny kinds of cacti, and not Christmas cactus.

Go through the checklist for conditions cacti like:

• Light—-must be direct sunlight at least 6 hours per day. Indirect light, light from fluorescent lights in the ceiling, and an hour or two in the morning are not sufficient.
• Water—-are you watering too much? At all? Large cacti might not need water at all during winter. Plants in 3″ clay pots might need water every 6 weeks. Never let succulents sit in a saucer of water, and don’t mist them.
• Temperature—-healthy plants can tolerate a fairly wide range of temperatures, but try to keep it 60–75 degrees F. Avoid cold drafts.
• Fertilizer—-don’t fertilize at all during the winter, and only sparingly during its growing season.
• Soil—-it’s best not to tamper with the root system unless you know that’s where the problem is. If there’s root rot from too much water or fertilizer, then remove the plant and tease away any loose soil and dead roots. Replant into a pot only large enough to hold the root system and no bigger. Use dry, gritty, sandy soil with little organic matter. Water sparingly until new roots grow, but it’s best to err on the side of caution and not water for a while.
• Propagate—-if the roots are dead, you might be able to save the plant by taking cuttings. Slice off the top of the stem and lay it on dry sandy soil. Keep it in the sun, and you might see roots beginning to grow in a few weeks.

Good luck!