yes, the pcb should be in a housing assembly . but, that is not to which i refer.
based upon your mounting description, there is a possiblility that conductive material is coming in contact
with the circuit board .
this contact is causing a current flow condition that exceeds the fuse rating causing it to open .
verify that your mounting scheme doesn't allow the components on the either side of the pcb to make accidental contact to any conductive surfaces.
qrz