I really sympathise with this, but would agree with buttercup and cheekygirl. If you've eliminated all the physical reasons why they might need to be awake (wet nappy, temperature, thirsty etc.), then it may be time to get tough.
Like cheekygirl, we've noticed that what used to be an occasional habit of waking and needing us to settle her, has now turned into something that happens virtually every night. It sort of crept up on us as she's had really bad eczema recently, and we've been going in to lather cream all over her to ease the itching. However, now that the ezcema is much better, she's still waking a lot.
She claimed to be thirsty (which is entirely possible with eczema as you lose a lot of moisture through the skin), so we were going in with a beaker. Eventually we started leaving the beaker in the cot, whereupon she'd start calling us because she'd completely emptied it. When we sorted that one out, she started losing bits of her dolly's clothing etc. etc. - basically anything to get us to go in.
The last few nights we've been getting really really tough with her - going in, but at longer and longer intervals, and being really really boring when we're in there. If it's at all fun, then she'll prolong it as much as she can!
We are also thinking about cutting back on daytime sleep, in case this is affecting her. She's 25 months, and still sleeping for up to 2 hours after lunch. Could this be the case for you?
Good luck!