OK so I figured it out. U gotta drag with your finger to attack the opponent and not the monsters on the field. So basically you're tapping on ur sword dragon and dragging ur finger to top of phone screen, that will make it a direct attack. Hope this helps some people
Don't know if phone user can do this too: As PC user when I click a monster(during BP), which is able to direct attack, a window open and ask if I want to direct attack with said monster. No need of dragging it to opponent(which sometimes glitch to monster...).
I tried more and more time to attack directly but does not work. Last example: Opponent with "Goggle Golem" (Earth) in Attack position, me: Alligator's Sword Dragon summoned. Can't activate the effect. Why?