What is the last character of the prompt, is it a # or a > sign?
Which show command are you trying to run?
When you are in the hostname> prompt you are in clish, this is where you can use commands like:
show interface eth1
show route
show configuration interface
etc.
if you are at expert@hostname:0# prompt you are in expert mode and you cannot use show commands, only standard linux commands will work here, like:
ifconfig
ls -l
df-h
ps aux
tcpdump -i eth1 host 10.10.10.10
but also check point commands like
cphaprob stat
fw stat
cpstat os
etc.
Hope this helps a bit.
Regards, Maarten