I have noticed that Pirates/Merchants spawn randomly. Meaning, if i take out a Merchant, it may spawn as a pirate in the pirate fields, or re-spawn as a merchant in any of the merchant spawn points. Same for pirates, if i take one out it may re-spawn as a merchant instead of pirate. I had this problem yesterday with merchants being gone, but had tons of pirates, took out a few pirates and bam, merchants starting spawning again.