Hello
System always checks the stock in warehouse stock location.There may be so many reasons for this kind of issues.
1. Check the routes of the products which are unavailable in reservation. If they are Make-to-order +'buy' or 'manufacturing' type products ,it will trigger purchase request or manufacturing order respectively to get that material.In that case it is required to receive them in STOCK locations.
2. The stock is already reserved for other MO.
3. The stock is available in other location than the reservation location of Manufacturing.Check the 'On hand quantity' tab of that product to check where the stock exactly in inventory.
I hope this will help!