Keep going, I have 4 accounts and even when I religiously refresh, I go through periods of not getting one on any account for long spells, can last a few days at times. Then again, sometimes I get quite a few in a short space of time too.
I find I get more on my account that does Expert difficulty encounters than those I have on hard or advanced difficulties. Not sure if the spawn rate really is better on higher difficulties or not, could just be a coincidence, but I think it is likely the case.
Just be careful as when they do spawn for you, unless you are playing on beginner difficulty, they will always spawn in low sec where players can scan and jump you.
Because you've mentioned you have all 4 mission types selected, I guess that means you have looked at the encounter mission filter. Do you have "Low-Sec" unticked by any chance? If so, that could be a reason why you haven't seen any appear. @bold nebula