Try using an interdental brush.

This is an interdental brush I bought at Daiso. I usually used to buy them at E-mart or Homeplus, but the Daiso ones are pretty good too. I visit the dentist a lot, so I take good care of my teeth now. I use various methods; I use dental floss and interdental brushes. You can't clean all the spaces between your teeth with a regular toothbrush.