It could be a few things, lets's start from the beginning:
I want to make sure your have the correct part installed. If you purchased the MLD-PC1, it came with 4 drippers and 4 Barbed connectors. How to figure out which one is the dripper, look through the center of the hole, if you see light, it is NOT the dripper, but the barb connector. The barb connector is ONLY for the purpose of connecting the 1/4" tubing to the 1/2" main poly line tubing.
Once you know which one is the dripper, I want to make sure you are installing the dripper correctly. The side that has a point is the side that goes into the 1/4"vinyl tubing. The flat side is the side the water flows out.
If you see that you have installed the dripper correctly:
When using drip, we recommend a pressure regulator to bring your household pressure down to 25PSI, (MLD-25PR). And we suggest a faucet filter (MLFF-41) it helps if you are pulling from a well, to keep out debris.
I would first trade out one of the drippers that works with one that does not. If the good dripper works, then the other bad dripper is clogged with debis and it cannot be cleaned out.
If after you switched the good dripper out for the bad dripper and the good dripper does not work, then it could be the water pressure, to much pressure leads to two problems. There is a diaphragm inside the dripper, once to much water hits this, it could have caused damage. If you do not have a pressure regulator installed, turn down the water flow at the water faucet, and see if that will help with this issue. Also, the 2nd problem if you have more than 25 lbs of pressure, it could start blowing out the jets right out of the tubing.
I hope this helps!