Hi,
proftpd has a few option that control timeouts:
TimeoutIdle -- Sets the idle connection timeout
TimeoutLogin -- Sets the login timeout
TimeoutNoTransfer -- Sets the connection without transfer timeout
TimeoutSession -- Sets a timeout for an entire session
TimeoutStalled -- Sets the timeout on stalled downloads
These could be (incorrectly) set or are not set at all.
Take a look here:
Proftpd - Configuration Directives for details.
Hope this helps.