No forum member eats,sleeps,stays on the forum 247,so most times ones post could go unnoticed either from merit sources or members loaded with smerits to give out,thus such posts gets buried without getting due rewards..
But when you make such posts consistently on the forum for weeks then there is no chance such user wouldn't be merited,so it's basically not a hit and run affair,except for the spammers looking for one golden merit...
If you're a good member you'll keep contributing your own quota to the forum and in the long run you'll get merited,and it wouldn't stop there as you'll keep getting merited as long as you're being of good help..
Tman's guide to getting merits
You can study this threads,they'll help you.