You don't need extra radio receivers unless other people are also firing flashes in the same room.
Flash sensors work when they detect a substantial and sudden increase in light, in other words when they see a flash.
Generally, this works well but there are several things that can stop it working.
1. As you've already discovered, a very low powered flash isn't always enough.
2. The increase in light needs to be substantial, and if there is either bright sunlight or the light from a modelling lamp falling on the sensor, the power of the flash may not be enough. In this situation you may need to block sunlight or turn off a modelling lamp that's shining on it.
3. You don't say which model of Bowens 500 you have. Where is the sensor on this one? Sometimes, sensors are in daft places. The best place is the top of the flash head, where it will 'see' the flash from pretty well any direction but sometimes they are on the back or the side and don't 'see' the flash unless it can reflect off of a nearby wall or similar.
The main light (the one with the radio trigger fitted to it) should always be fitted with a broad light source such as an umbrella or softbox, which will spread the light around the room and hopefully catch the sensors on the other flash heads. If the main light is fitted with a narrow light shaper instead, such as a honeycomb or a snoot, it may not set off the other lights.
