Not Getting Completion Credits Towards Achievement

My thread title conveys most of my issue, though more specifically, I’ve been playing Halo 5 for a decent amount of time now, and I’m at least 95% sure I’ve played all of the unique game types found within “Big Team Battle” at least fives times each, and am at least 80% sure that I’ve won that many games within at the very least slayer and strongholds as well, though I only have 3 credits towards “Spartan Decimation”, and none towards “Flag Monger” or the related strongholds achievement.

I was thinking the issue might be that I might have to complete those game types within a specific playlist, though I tried at least a couple games within “team slayer” and as far as I recall I didn’t get any credits after playing those either, and I’m also pretty sure strongholds is exclusively tied with big team battle as far as matchmaking goes too, so I didn’t think that hypothesis would be very likely.

Some help/clarification here would obviously be appreciated, as while for the most part I am having fun playing Halo 5, I don’t want all that time playing to be in vain when I could be earning achievements as well. Provided I get some help here, thanks in advance!

P.S. I didn’t look through FAQs to excess, so (provided they exist) I apologize if I was meant to, I just thought this method would be more expedient. I also wasn’t sure if this topic was best suited to being encompassed under “general” or gameplay", though gameplay made a bit more sense to me which is why I chose that category.

Try playing Team Arena for a while. You get a mix of slayer, strongholds, capture the flag etc. That’s where I completed those achievements, although that was a good while ago now.