Some outbound calls are recorded and others are not recorded

Hello,

Currently, i have got problem which some outbound calls are recorded and others are not recorded. .
This i can see when want to playback the recording via GUI. Some conversations/calls appeared the speaker/audio and disk icons on the Monitor column.
My server have 2 GB RAM, Processor Intel Core2Duo 2 GHz, HDD 500 GB.
The total users that always record are 8 extensions (SIP Phones), trunk is DAHDI (FXO)
Can someone help me?