just to close things off, the solution was to use the right resistor values.
did not know about having to use certain ohms depending on the capacitance of the bus.
When I was using one device, I had the right resistance and it worked.
using two devices, the capacitance changed, but the resistance stayed the same, which did not let the bus function properly.
the more devices you have, the less resistance you need.
Hope this will help someone who got stuck like i did