Should I submit my paper to a different journal after not receiving any acknowledgment for five months? I submitted a paper to a low to mid-tier online-only journal in my field last January. I did not receive any acknowledgment from the editors that they received my paper and the journal has already published papers that were submitted after I submitted mine. Having been busy since January I forgot to ask the editors for an update. I was only able to send an email a week ago, and I still did not receive a reply. Should I submit my paper to a different journal or should I wait a bit more for a reply?
Give them another week, then decide.

Sending two messages within one week may be considered as pushy, I would try to avoid this. However, if you have serious concerns about the status of the paper it may be justified nevertheless. Whether or not your concerns are serious depends on the history so far: if you have not received any notification at all (via any submission system or similar), then maybe the paper actually got lost.

Personally, I would have contacted the editor earlier since in my field the review duration is 2-3 months -- of course, this may be different in your discipline. Some journals state certain statistics on their web page, such as the time from submission to the first decision. This gives you a hint what time spans you can expect until your paper is reviewed.