Did everyone in your team actively ring a bell? Cos even if you're in a fireteam, unless u actually ring a bell it won't count. Could also maybe a delay for your achievement... i hear sometimes you'll get an achievement hours to days later after completing it. Idk maybe try with 1 other friend so there's less people in your team.. idk? I did mine with a friend.. got him on bell 1 and I rang bells 2-5 and achievement unlocked no problems for the both of us.