Baby not sitting yet

My baby just turned 7 months old and refuses to sit independently. Every time we try he straightens his legs and leans backward so we can’t sit him down. He has sit independently before for short periods so it seems more like a behavioral thing than anything. He’s also pretty chubby so I think it’s more difficult for him to support his own weight and he doesn’t want to try. He’d rather just jump in his bouncer 🤦🏼‍♀️ Anyone else in a similar situation?