Vitaly Janelt had the Bees ahead before Harry Wilson scored twice in injury-time for Fulham's 2-1 win.
Frank later said: "I think the better team on the day won it in the end. We lost it from a situation where we did everything right - credit to Harry Wilson but he only scores from that position one in 100."
When asked whether it would be fair to say Brentford didn't match their usual level, Frank said: "That would be more than fair. Every game has its own story - we mainly look at ourselves, but there's another team on the pitch who did well today.
"Let's look at the end of the season and see how many wins we've got on the road.
"On paper Fulham probably aren't the biggest team we've played, but they probably did the best against us in terms of dominating the game."