On a dummy example does it work for me. Verify your start/stop times in the panel. Also verify the time step size used to advance the particle(s).
In my example Particle is first introduced at 0.0, my time step size is 0.001[s] and the repeat interval is set to 0.005 [s], every 0.005[s] I get the one particle injected. So my second is injected at the beginning of the 6th time step and so on..