Have you got males?
How calm is the surface?
Any floating plants on the surface?
How often and what do you feed them?
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Labyrinths build bubblenests on still water. If there is too much aeration/ surface turbulence it breaks the nest up. Having floating plants like water Sprite help to clam the surface and give the bubblenest somewhere to attach.
If the fish are not fed enough, they won't be in breeding condition and won't want to build a bubblenest. Breeding fish should be fed 3-5 times a day with a variety of dry, frozen and live foods. If you don't want them to breed, just feed once a day.
Do more water changes and gravel cleans if feeding more often to help keep the tank clean.
Make sure the temperature is warm enough for them.