You get more plaque buildup around the lower front teeth because...

...saliva around the lower front teeth has higher mineral concentration. This prevents[1] tooth decay but makes plaque harden more quickly. When the plaque is hard you can't brush it off.

Solution? Cut the time between brushings to catch the plaque while is is still soft.

[1] OK, maybe not 'prevents', but 'greatly reduces'. Very few people have decay in their lower front teeth.