Ur sub may have one of those auto-shut off feature. DISABLE IT.
How to disable it ? i couldn't find it on sub woofer. The subwoofer having following switches,
Phase Switch Theres is 2 option. Reverse and Normal( i put "reverse")
Bass Boost Switch (off)
Power On Mode Switch. Auto / On (On)
Subwoofer Level Control
Input Mode Switch Normal / LFE (Normal). If i choose LFE the boom sound coming more loudly.
External Trigger Input Connector
Line-Level LFE In Connector Line-Level L/R In Connector (Connected cable from AVR to LFE port)