Why has my baby stopped falling asleep in the pram?

My baby has always reliably fallen asleep in his pram when we are on a walk (usually waking is we stop walking). In the early days as a newborn it is the only way I could get him to nap during the day. For the last 3 days he won’t fall asleep in the pram and gets very grouchy around the time he needs to fall asleep. As soon as I get home and try to put him to sleep he conks out in minute. What has changed? Only a few weeks ago I did all his naps in the pram! Has anyone experienced this? He is 5 months old.