I've carried both of my babies in slings from birth and never had a problem until now!
My DS is 12 weeks old and will only nap in the sling. I've tried various other places, bouncer, being held, pram, his crib, and he only sleeps for 20 mins max. But in the sling he will sleep for around 1.5-2hrs between feeds.
The problem I'm having now, is that he fights and wriggles and thrashes when we are in the house! If I'm out and I put him in the sling, he will be asleep in minutes few, but in the house, he freaks out the second he goes in the sling! He will thrash and scream for a good 15 mins, sometimes more before he realises "oh, it's actually quite nice in here" and goes to sleep 
Any ideas what the issue could be?! I've made sure the house is at a comfortable temperature and his clothing reflects the temperature, inspected the sling to make sure nothing could be pinching his skin or anything like that and I can't see anything!