I have the exact same problem with my Scarlett 2i2. I have the original iRig adapter and also the iRig HD. The original iRig adapter actually has worked well once you fight the feedback and start dialing levels down, but the iRig HD has been a paperweight for me. I actually have it boxed up ready to return it to Best Buy
This probably won't help but over the weekend I had the same exact problem, it ended up being the cable.
Now I have not tested the cable since but I am going to because I used it one time in total.
I ended up getting a 1/8 to DUAL 1/4 and started using that and put the "bad cable" away.
I have been having very odd issues with BIAS and recording that I can not wrap my head around.
For example, Sunday night I was messing around, trying to get a good sound.
Everything was going well. I took a quick smoke break, came back 10 mintues later and for whatever reason there was minimal output. A huge difference in volume and you could even see it on the actual track waves.
I have had the same issue "volume drop" when using bias with my live/practive rig. Never could pinpoint it. Very big PITA but hey what do you want for $10