I often see infants aged between 11-15 months who have learned to sit but are not rolling, hate being on the tummy and are not crawling. Parents are understandably getting anxious about their infant's development and may have been advised to work on tummy time. 

Trying to get these infants to spend time on the tummy is an uphill battle and mostly leads to refusal and tears. So that is not the way to go. Personally, I never get into a fight with an infant - they always win. 

So if your infant is happy sitting, the better approach is to start by getting the infant to be more active in sitting and working to get him/her to move from sitting into lying and up again as well as from sitting to kneeling positions. You can also start working on pulling up into standing. As your infant gets stronger and gains more confidence he/she will start to try out new things and put more effort into diffiuclt tasks. 

More active in sitting - stretching and twisting 

Moving from sitting to kneeling 

Standing up from sitting on a low block