AFI 31 week pregnancy. Down position.

My afi is low at 31 weeks, it is 6.3cm. 3 weeks before it was 9, that time my dr was saying it is in border line low and will increase with more water and fluid intake. I drink 4lts water daily and also coconut water regularly. Dr also prescribed me medicine for 10 days. After 3 weeks it reduced to 6.3. I am very worried now. Now my dr is saying to take fluid in hospital as day care for 3 days and also prescribed medicine and powder to increase it. My question is, is there any way we can increase this and what can be cause of low afi. My baby's position is still head up, weight 1.6 kg and not affected with this low afi as per my doctor. Please let me know what can be cause of low afi, how can we increase and is there anything serious if my baby is not in head down position. Thanks