Quick answer
Often it's just early days: colour builds over several sessions. But some skin simply doesn't tan well and burns instead, and for those people UK guidance says sunbeds aren't suitable. Old lamps, dry skin and long gaps between sessions can also slow things down. Don't respond by staying on longer: that raises your risk of burning without improving your tan.
Possible reasons
It's early days
Colour usually builds gradually over several sessions. See how many sunbed sessions it takes to get a tan.
Your skin doesn't tan easily
Very fair skin (type 1, and many type 2s) tends to burn rather than tan. If that sounds like you, UK health guidance says not to use sunbeds, because the UV mainly causes damage. See what's my skin type? and who shouldn't use a sunbed.
The sunbed
Lamps lose output as they age, and beds vary a lot. See sunbed minutes and lamp strength explained.
Dry skin
Dry, flaky skin sheds colour quickly, so it can look as if nothing is happening. Moisturise daily and consider a sunbed lotion: see do sunbed lotions really work?
Washing it away
Long hot showers and scrubbing fade colour fast: see how to make your tan last longer.
What not to do
Don't increase your session time beyond the salon's recommendation, or add extra sessions close together. It won't fix the problem and makes burning more likely. If you'd like colour without more UV, a bronzer lotion adds instant colour: see accelerator vs bronzer vs tingle.
