Question / Help Pops and clicks when using Focusrite interface !

This is odd !
I run two audio interfaces on the same computer. I use them to DAW ( Digital audio workstation ), I record professional audio. My interfaces are a Focusrite Pro 40 and an old EchoAudio Layla 24. I run Sonar X3 as DAW, no problem ( I use only on focusrite ). When I use OBS, I need to use the Layla audio interface, because if I try to use focusrite one as input/output ( even when not using for other purposes ), I get pops and clicks from time to time. My PC is intel i7 2600k running at 4.8Ghz, 16Gb RAM, SSDs... I run 2 video capture cards, a blackmagic PCI 4K, and Avermedia 1080p. If I use only the echoaudio interface, no problem, but I need to use focusrite, which is more complete and modern. What´s the problem ?

Thanks any help !