You need to add larger rougher rocks and stiffer plants. Assassins really like those surfaces to lay their eggs on. I use Swords and Bog plants because they are stiff as a place for them to lay. I would also add MTS to the tanks and go with sand. Baby assissins like to burrow and in that large gravel you may not even know you have baby assassins until 2 moths after they hatch because they will live below the large rock substrate until they are bigger.
I have not had much luck with them laying the eggs on the bog wood and with the larger substrate rocks they will lay them on the substrate instead of the plants. Also you will not be able to siphon that tank once they start laying or you are likely to suck up eggs or fry.
Once you see the first egg it will be a 3 to 4 week wait for babes.
How big are your Assassins... I have found they do not start to lay until at least 3/4 of an inch and you want to make sure you have at least 5 in the tank since they do not pair up but one male will be with several females.