If  you are experiencing low suction because breast shield is too large,  you may consider to purchase little breast shield to convert the size of breast shield from 28mm or 24mm to  26mm, 22mm or 18mm.
 
18mm & 22mm little breatshield can be inserted below 
24mm breast shield
24mm silicone breast shield of THE INs and Express Cups
 
26mm little breastshield can be inserted below
28mm silicone breast shield of THE INs and Express Cups
 
Made from silicone