Newbie needs help - Korean (Kalbi) Ribs

hi. i followed the amazing ruler guide and had 5 bags with 4-5 'flaken' cut short ribs single layer (preseasoned with little marinade) and set the Anova at 131 degrees F for 3.0 hours. Then i removed and placed under broiler for 3-4 mins. for charred effect . flavor was fine but the meat was not as tender as i expected. some of it was chewy . none just fell off the bone. it came out just like how i normally cook under the broiler....
so - i'm thinking i might need to invest in a rack so that the bags can be separated and provide better water circulation? or was it not enough time in the water bath?
please help....thank you!