It could be all sorts of things, are they loosing their dummy and waking, is it too cold in their room at night? Is it teeth? Could be a developmental stage where they're waking a lot due to crawling in their sleep etc.
Sometimes with mine I find if I adjust their nap routines in the day and wake up and bedtime, all of a sudden they sleep better at night. That's worth a try.
My son whose 13 months, I wake at 7am, naps at 10.30am and 3pm in the day and is in bed again at 7pm. Make sure he eats plenty in the day and drinks lots too. I find the worse he sleeps in the day, the worse he sleeps at night. Bizarre but true. So naps are really important to him in the day, or it throws everything off track.
If he is ill or teething he does wake more at night and I may have to give him teething gel to calm him or worse case scenario a bottle if really distraught - BUT most of the time he is a good sleeper now. He's a thumb sucker.
It's a shame not all babies are the same, as my son is soooo different from his sister, it sometimes means all your previous tricks do not work
this book I find helpful, the author is a child's sleep expert and explains developmental stages, common problems and solutions etc in his book, right up to adolescents! I don't use his sleep training, I prefer the baby whisperer PUPD method but his advice is usually spot on. If you have the time to read it with 4 kids!
Either way hope things get better soon x