With the big league club faltering this season, some Houston Astros fans may switch their gaze to the minor leagues for reinforcements. The Astros are flush with young prospects, but not many are big-league ready. Now that the minor league season is well underway, let's take a look at which top prospects are struggling to this point in Houston's system. Walker Janek, C, Double-A Joe Rondone/The Republic / USA TODAY NETWORK Walker Janek is the Astros' top catching prospect by a long shot. The next highest-ranked catcher in the organization was drafted last season out of high school and has played just one professional game to this point. That doesn't mean Janek is the only catcher who can be promoted, but that's probably not on his mind right now. Houston's No. 4 prospect has batted .135/.179/.216 through 19 games this season and hit just one homer. His biggest flaw thus far has been his BB/K ratio. Through 74 at-bats, he's struck out 28 times and walked just four.…