My poor buba

So today at sensory class I realised my little guy was the only one who who couldn't hold his head up on tummy time or push up onto his chest he does it for like 10 seconds then face plants the floor and starts licking it...yes he licks everything...but I felt really bad for him. Some babies younger than him were managing fine...He's 5 months old why hasn't he cracked this yet? He can hold his head just can't seem to do it in that position for very long...