UPDATE: After some further testing, we're seeing issues with Modified decks that are using basic Energy cards from sets prior to Black & White. We're investigating this matter further and will update the community when we have more information.
Since the introduction of Play Types this week, we've seen a number of comments in the community about an inability to find an opponent in Player vs. Player. We believe this is a critical mass issue (number of people looking for a match at the same time), rather than a bug in matchmaking. Every theory or solution presented by the community has been tested by the QA team and none have proven to be the cause.
Prior to the introduction of Play Types, players searching for a match fell into one of two buckets: Novice Mode and Expert Mode. Now, players fall into one of six buckets: Unlimited-Novice Mode, Unlimited-Expert Mode, Modified-Novice Mode, Modified-Expert Mode, Theme Deck-Novice Mode and Theme Deck-Expert Mode. With the same number of players split across six game modes, the likelihood that someone is looking for exactly the same type of match as you is greatly reduced. We're working on improvements to matchmaking to mitigate this split.
As always, if you believe that you have found a bug with matchmaking, please submit it via the forums or a support ticket.