I will try and provide as much information as I can and I apologize if I call something by an inappropriate name – but this is what I have done from my research.
We had a room that had a central light that was was able to be switched on/off at the two entry points to the room. I wanted to add four recessed lights to the room and just leverage the same switches, etc. However, when I went to connect all the wires the LEDs are very dim. I am able to remove the one switch from the circuit and the lights function to their full capacity with the one switch functioning properly. When wired to the other switch (removing the first from the circuit) the lights and dim and the switch that is connected does not work. I have attached some crude drawings in an attempt to show the wiring better as I am not sure how clear my words are. Let me know if I can help to clarify any details.
Best Answer
This can't work without smart switches
You can't wire 2-way switches with only twin-and-earth, unles you do something totally evil and dangerous. If it ever worked before, it was wired horribly.
You are groping around trying to experiment, which you should not do, but at least you are using the wires properly - reserving green/yellow for ground instead of bootlegging a hot or whatever horrible thing the last guy did.
The runs from lamp to switch need to be replaced with 3-wire + ground (whatever they call that where you are). Then we can use 2 of them as what we call "travelers" and have the third for hot/common.
Your alternative if you can't rewire is to use smart switches, but this will require a module in the first lamp.