While playing today my infant got the sit up milestone. However the game would not progress further. This is my first time having the bug since the update and getting new packs. The infant got the si...
Ideas
Try switching your active household and then going back to the original household. The infant should be able to achieve the milestones then (to note...usually if you travel with a sim from your house to a different lot and come back, the infants stop achieving milestones until you change active households or restart the game altogether). The bug is super annoying and apparently EA isn't prioritizing fixing it since it's been an issue for years, but that's the workaround I've found.
Thank you so much about the cause being travelling, I did notice that on restart of the save the infant began getting milestones again (sit up unlocked). Today mother had to travel to the hospital for birth of new baby and my infant stopped getting milestone again, (the finger food one) so I switched household like you said and came back and he got the milestone. 😀
