It is maybe because Transport Mute is engaged.
It mutes the signal immediately when your DAW is paused.
When playing a soft synth on a track that also has Efx FRAGMENTS, that the DAW is playing, and that your synth produces no sound, disengage Transport Mute to remedy the situation.
In the Grain Quantization section of the effect, you can more precisely control where the play head sources grains within the buffer.
If out of the three options you choose transient, grains will be generated at the transient closest to the play head at any moment.
If there is no transient, there is no grain.
There are two main reasons explaining why you might hear only a few grains.
1. The value you have assigned to the density parameter is too low.
By setting a low density, the grains produced within a timeframe are scarce.
2. The value assigned to the Maximum Grains parameter is too low.
You can remedy this by dragging up the value of this parameter.
To take back control and have more subtle results with Efx FRAGMENTS, you can set lower values for density, feedback and size.
1. Density
By decreasing density, the number of grains produced within a time frame will be reduced.2. Size
By decreasing the size, the duration of the grains will be reduced.3. Feedback
And finally, by decreasing the feedback, you can reduce the amount of feedback of grains.The collar around large knobs randomizes the setting's value.
It is then really important to distinguish between the inner collar, which sets the actual value of the parameter, and the outer collar (yellow collar).
1. Spray
For spray, it randomizes the start position of the grains.2. Randomize density
For density, it randomizes the rate at which grains are generated.3. Randomize size
For size, it randomizes the length of the grain.4. Randomize pitch
For pitch, the yellow collar helps you set the amount of random pitch.
Note: Depending on the mode you choose out of the three options in Grain Release, some yellow collars might be bipolar, meaning that it can add positive or negative randomization amounts - or both at once.
When this happens, simply mouse anywhere over the buffer display and you will see four icons pop up at its top left corner.

By clicking on P. Retrig (PlayHead Retrig), the buffer play head will start at the beginning of the buffer at each new cycle, as determined by the buffer length setting.
Efx FRAGMENTS is intended for use on stereo tracks, but you can still insert it on a mono track.
It will then be using "mono to stereo". It means that you can feed it a mono signal, but your output will be stereo.
This information might particularly be useful to users of Logic and Pro Tools.
Each grain can be pitched up or down.
Pitching up means playing grains faster compared to the recording head.
It means that if your buffer is empty, there will be no sound since there is no audio before the recording head.
After filling an entire buffer, grains can be generated pitched up.
Instead of waiting, you can also deal with this issue by setting an appropriate value for the offset in the Grain Capture section.
Note: The offset parameter sets the offset between the recording head and the playhead.
When switching between Classic, Texture and Release, the values set for the parameters are kept, except for Layers in the Texture mode.
When you hear a lot of clicks coming from the grains, and you don't like, please check the grain shape you've chosen, and check what value you have assigned to this parameter.
You can then either change the grain shape, and/or increase the value of this parameter.
Notes: Some particular grain shapes are more prone to clicks, like Expodec.
When changing quickly the tempo, it may cause issues with your buffer.
One way to deal with this is to clear the buffer first, and then change your tempo.