Round 13: Mega Ampharos / Skarmory / Clefable / Talonflame / Hippowdon - Because Talonflame is on the team, there is only one option for the final member: a hazard remover (Skarm lacked Defog here). The team also lacks a bulky water, and Starmie is the undisputed ideal choice to help with threats like Heatran and others. Starmie ended up winning. The actual final member was Tentacruel if I'm not mistaken, but I only know this because the team was actually posted in another OU forum project prior to the day of this round.
Round 16: Mega Lopunny / Hoopa-U / Landorus-T / Rotom-W / Latios - The team lacks a steel. The obvious conclusion is the final member is a steel type to help the team deal with fairies and threats like Latios. This only leaves very few options to realistically choose from for this team.
Round 17: Skarmory / Mega Gardevoir / Tyranitar / Keldeo / Gliscor - It's well known how these teams are constructed. The team relies on Keldeo as the water resistance and a grass type is the natural choice as the final member to patch this and other issues up. This, again, only leaves so many options to choose from, with only three submissions ending up posted during this round.
Round 18: Tangrowth / Heatran / Mega Latias / Landorus-T / Terrakion - A well known team at the time of posting, with the actual final member being Hoopa-U (who was banned then). Realistically, a water type like Keldeo or Rotom-W is the only viable option for the remaining member with Hoopa-U banned. This doesn't leave many options to pick from, and the submissions for the round reflected this.
Round 19: Jellicent / Skarmory / Hippowdon / Excadrill / Clefable - The team has some clear weaknesses, such as Charizard-Y, reliance on Jellicent as the sole water resistance, difficulty breaking through opposing Skarmory, and trouble with grass types like Mega Venusaur and Serperior. The team also lacks a mega. Put this together and you only really have one or two realistic options that can viably help with all or some of the issues: Mega Latias or Charizard-X. This also lead to a lack of submissions.
Round 20: Charizard-X / Mamoswine / Serperior / Latias / Cobalion - This team has a lot of weaknesses that I won't go into, but the most evident ones are Talonflame and a lack of a reliable answer to threats like Excadrill. The team also lacks a water. Only one Pokemon can address these issues and others while fitting the team's nature: Rotom-W. This ended up winning, and other submissions aside from Rotom-W didn't really stand much of a chance.
Round 21: Mega Houndoom / Amoonguss / Landorus-T / Jirachi / Latios - Lacks a water, which any offensive team like this one will likely have. Like other posters mentioned during this round, Rotom-W is the only real option for the final member to patch up several weaknesses (like Talonflame) and give Mega Houndoom chances to come in freely.
Round 23: Mega Venusaur / Heatran / Doublade / Starmie / Clefable - The currently ongoing round. As others already mentioned, the biggest issue is a lack of a ground immunity and weakness to things like Bisharp. This realistically leaves very few options, with Gliscor (my guess as well) being pointed out very quickly. From there, there's not much room for other submissions to compete.