The inserts to your brand new cloth diapers should be washed before they get the maximum absorbency. That’s the way it is with natural materials. You probably know it from new towels that don’t absorb well at first.
Bamboo contains natural oils that need to be washed out before the absorbency is optimal. This can be done by soaking the new inserts overnight in cold water. Then you can wash the diaper shell and inserts as normal. You have to expect to change the diaper frequently at the beginning until the inserts have been washed about 8 times.
The inserts are going to shrink slightly after washing due to the natural fibers. You can advantageously stretch them in wet condition to the maximum size.