why does my qfx3500 fan fail every 1-2month ?

Hello,

I have a 2x QFX3500 in virtual chassis mode and every 1-2 month one of my Fan trays showed as failed and everything is ok , Fan trays are spining at normal speed they have 1unit free space from up and bottom of the every switch, and here is the switch output :

Class Item Status Measurement
Power FPC 0 Power Supply 0 OK
FPC 0 Power Supply 1 OK
FPC 1 Power Supply 0 OK
FPC 1 Power Supply 1 OK
Temp FPC 0 Sensor TopLeft I OK 21 degrees C / 69 degrees F
FPC 0 Sensor TopRight I OK 22 degrees C / 71 degrees F
FPC 0 Sensor TopLeft E OK 35 degrees C / 95 degrees F
FPC 0 Sensor TopRight E OK 33 degrees C / 91 degrees F
FPC 0 Sensor TopMiddle I OK 31 degrees C / 87 degrees F
FPC 0 Sensor TopMiddle E OK 31 degrees C / 87 degrees F
FPC 0 Sensor Bottom I OK 37 degrees C / 98 degrees F
FPC 0 Sensor Bottom E OK 30 degrees C / 86 degrees F
FPC 0 Sensor Die Temp OK 42 degrees C / 107 degrees F
FPC 0 Sensor Mgmnt Brd I OK 21 degrees C / 69 degrees F
FPC 0 Sensor Switch I OK 31 degrees C / 87 degrees F
FPC 1 Sensor TopLeft I OK 21 degrees C / 69 degrees F
FPC 1 Sensor TopRight I OK 20 degrees C / 68 degrees F
FPC 1 Sensor TopLeft E OK 27 degrees C / 80 degrees F
FPC 1 Sensor TopRight E OK 27 degrees C / 80 degrees F
FPC 1 Sensor TopMiddle I OK 27 degrees C / 80 degrees F
FPC 1 Sensor TopMiddle E OK 34 degrees C / 93 degrees F
FPC 1 Sensor Bottom I OK 33 degrees C / 91 degrees F
FPC 1 Sensor Bottom E OK 33 degrees C / 91 degrees F
FPC 1 Sensor Die Temp OK 41 degrees C / 105 degrees F
FPC 1 Sensor Mgmnt Brd I OK 19 degrees C / 66 degrees F
FPC 1 Sensor Switch I OK 27 degrees C / 80 degrees F
Fans FPC 0 Fan Tray 0 OK Spinning at normal speed
FPC 0 Fan Tray 1 OK Spinning at normal speed
FPC 0 Fan Tray 2 Failed
FPC 1 Fan Tray 0 OK Spinning at normal speed
FPC 1 Fan Tray 1 OK Spinning at normal speed
FPC 1 Fan Tray 2 OK Spinning at normal speed

show chassis alarms
1 alarms currently active
Alarm time Class Description
2020-11-06 04:44:17 UTC Major FPC 0 Fan 2 not spinning

the strange part is Fan Tray 2 LED is ON and its not off and when i put a little tissue paper on fan tray 2 it seems the fan tray is working but i do not know why does in qfx it shows as failed,

i have reboot whole of the switches but the issue still exists,

here is picture of the switch : Image may be NSFW.
Clik here to view.