Hi! My 5 months old keeps rolling on her side to sleep, we put her down on her back but she keeps rolling on her side after she fell asleep, even after we keep rolling her back onto her back, she just refuses to stay on her back while sleeping. She can roll from her back to her tummy but she can’t roll from her tummy to back yet. She can roll both directions ( to tummy or to back) from her side. Is this a concern? Should we keep her on her back during sleep?

Most babies will be rolling over by 4- 5 months…...some earlier, some later. 
This is normal as babies grow & become stronger.                         
Many babies then prefer to sleep on their tummy or side.
This is not something that needs to be stopped. It is important to make her environment as safe as possible.
Rolling for babies is a bit of a process - like sitting/crawling/walking it takes time to be able to do well.

When baby attempts to roll & looks like being successful, then the following is appropriate.
Move baby from bassinet to cot - baby needs space of cot to roll well.
Cease swaddling/wrapping - baby needs arms free.
Increase tummy time when awake to continue to build upper body strength.
** Always place baby on back for sleep - even when rolling.
Ensure mattress is firm, well-fitting & flat (not elevated).  - a soft mattress may increase the risk to a baby who is starting to roll.
Nothing loose or soft in the cot.

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

By 4-5mths most babies will be rolling & prefer to sleep on their side or tummy - eventually they will be rolling both ways well.
At this time, you can leave baby sleeping in their preferred position.
There is nothing wrong with rolling baby back from side or tummy, but eventually many will resist & roll back to their preferred position.
You are not expected to be awake all night to turn baby back, but continue to always place baby on back for sleep, in their own safe environment.