In my opinion several factors contribute to our current lack of Champions League success.
1, seedlings/pot placement. We have had to contend with so far one outstanding team and one difficult team in the group. Bayern twice and Madrid once, Napoli, Dortmund and to a lesser extent CSKA although weaker, still a difficult away game. If we can avoid a European super power, top the group then extended participation is likely. If we'd have Chelsea's group & draw I'd have fancied us to do as well if not better.
2, Squad. As our continued involvement in the CL seems likely, I expect our squad to adapt. The likes of Lescott, Barry, Richards will be replaced by either top quality young talent or seasoned CL performers. Our squad is good, but fringe players lack the required experience.
3, Luck. A favourable draw, good officiatiopng in specific games and our key players remaining fit. I watched Athleti beat Barcelona, they got in their face, imposed their physical superiority over them something we couldn't do. The ref in that game allowed things to flow, something we all know we couldn't/weren't allowed to do.