Yeah it very well could be undercharged that's why I said start by adding some refrigerant and see if that helps since it's easy to do. Do that with full airflow though. If it doesn't help then you just need to shorten your cap tube a little bit. The higher pressure on high side from blocking the airflow will push more refrigerant through the capillary so it is very likely your cap tube is just a little bit too restrictive.