Likely either too long a run or too small diameter of the supply line.

I appreciate the reply.

The setup is 3/4" PVC pipe underground with 1/2" poly tubing/drip line stemming off of the PVC, which I think is fairly standard. And this drip line is rated for at least 300', so I don't think that's the issue if I've only run half that so far.

Several factors apply more than length and size of supply line. What is the source pressure/ What is the hole spacing in the dripline and flow rate per foot? What is the slope of the ground which the dripline traverses? Type of plants? The 3/4" line will be too small for a 300' length. In my experience you will need two driplines to go the length to shorten irrigation time and water coverage of the root zone (plume). You have to design for mature plants with large root zones.

After finishing the entire installation of 300' of drip line, several emitters were simply not dripping at all while others were flowing freely. My thoughts went back to the discrepancy I witnessed in the amount of flow, so I picked up a shovel and started digging at the point where the supply hose came up from being buried. I followed the line a ways to where it disappeared into the root ball of a 15-year old jasmine bush. I hated to do it, but I dug out the jasmine. Sure enough, the supply line had been kinked. Once I straightened the line, you could hear the drip line emitters hiss throughout the installation, and I had excellent and, most importantly, consistent flow from each emitter. Problem solved! I was sad to see the jasmine go, but it was for the good of the rest of the garden.

Thanks to everyone who offered advice. Happy spring! :D