For a start, show how your cams and oscam are configured and post logs when working and not working.
for tvheadend --trace caclient,csa,capmt,cwc,cccam,dvbcam,descrambler,en50221 (so 4 logs)
and for oscam --debug 9 (two logs concurrent with tvheadend logs)
also output of "ls -al /dev/dvb/adapter*"
Maybe you need to change your device nodes to match the redirection???