Does this system support Procedural Simulation?
Internally, each Particle System has 2 modes of operating: procedural and non-procedural.
In procedural mode, it is possible to know the state of a Particle System for any point in time (past and future) whereas a non-procedural system is unpredictable. This means that it is possible to quickly fast forward (and rewind) a procedural system to any point in time. This can be particularly useful if you are perfoming custom simulation in script, such as rewinding an effect.
In order to support Procedural Simulation, you can only use a subset of the Particle System modules and properties. For example, using the Limit Velocity over Lifetime module will disable Procedural Simulation. Additionally, modifying any properties from script whilst the system is playing will also disable Procedural Simulation.
To discover if you are using any properties that disable this feature, a small speech bubble appears in the upper right corner of the Inspector. The tooltip for this icon gives you details about why Procedural Simulation is disabled.