I took out the whole assembly, it wasn't the spring, there were sound on the actuator, so I thought
it must the something broken in the actuator unit. I opened the actuator housing (though difficult)
and found the plastic gear driving the actuator broken.
I glued it back, reassembled and it is back in business
BTW, anyone has a faulty actuator to spare? Don't know how long the glue will last though!