Hi
It's a little hard to say - knowing so little about your setup. Perhaps you could describe it a little more.
Following could influence:
1) Age of fishes (very young - very old). Probably only fish in their best age will produce the big spawns.
2) Their condition (sufficient and good food)
3) "Temperament" of the father (eating eggs) or other males as stressfactor
4) Water Quality (perhaps an UVC could bring something. I use it myself - some other breeders too. You dont have to use it permantly - just while spawning / hatching and so on). Oxygene, current is of course always worth paying attention.
5) What kind of filtration do you have? An very old one could also influence negatively.
6) What kind of bottom? Bare, gravel, sand? Everything healthy here?
Just a couple of ideas. Perhaps you get inspired
I have on my site 76 spawns registed till now - totally 670 fry - the biggest spawn with 21 and an average of 7 ! So you are lying quite normal
Best regards
Line