You're not running enough or you're not running strides.
Most runners think to get faster you have to run a lot of fast workouts. But most runners simply need to run more because what they really lack is endurance. You can probably run 6:00-per mile pace for a few seconds. So the solution is to increase your endurance so you can hold that pace for longer.
More: Boost Your Endurance in 7 Simple Steps
Start by increasing your long run by a mile once every two to three weeks. Runners training for a 5K or the marathon should all be doing consistent long runs—it's your most important workout of the week!
Your weekly mileage will also help boost your aerobic capacity so try to increase it by about 10 percent every two to three weeks until you've reached your "baseline workload" (here's how to plan training that's just right).
More: How to Increase Aerobic Fitness
Strides should be a regular staple of your weekly running program. They're 100-meter accelerations where you start at a slow run, build to about 95 to 98 percent of your maximum speed, and then gradually slow to a stop. One stride should take you about 20 to 25 seconds.
Start with four strides, and after about three weeks, increase to six per session. Take about a minute of walking or standing in between to catch your breath. Strides are not an aerobic workout so don't rush—you don't get additional benefits by reducing the recovery period.
Strides are short and you're only running fast for a few seconds, so they shouldn't be very difficult. Remember to stay relaxed—at no point should you be straining or sprinting.
So take an honest look at your running over the last three to four months. It doesn't matter what your goal race is, these upgrades to your training can help you accomplish that elusive PR you've been chasing.
More: How to Train for a PR
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