I have "American Standard Vent Away" wall hung commodes (3). They were converted to standard flushing mechanisms years ago. One of the commodes flushes sometimes and other times not. I'm sure the drain lines to the septic tank are clear. The tank was pumped last year. This particular commode has all new internal tank parts and is adjusted to fill the tank to the proper level. About half the time it requires a second flush or more. What is the corrective action to take?

When the toilet fails to flush properly, can you describe in detail what happens? This usually means a partially blocked drain from the toilet.