general guidelines:
first aid to see what's up with the packet use 'fw monitor'
for example let's filter some dst (9.9.9.9)
fw monitor -F "0,0,9.9.9.9,0,0"
run ping to 9.9.9.9 from the PC behind the Spark.
you should see the same packet four times, with i, I, o, O.
if you see only i, it probably drops. (run fw ctl zdebug + drop to see the reason)
if don't see o, you don't have route (probably not the case)
on O, you should see the source is changing to the NAT IP (rather then original PC IP).
if you still see the original IP, you don't have NAT.
how exactly did you configure your NAT ? (Automatic NAT/manual NAT) please provide screenshot or details.
Thanks