To be fair it's not like Reverse Flash was standing straight up and staring at Oliver as he shoots the arrow towards him and he doesn't dodge it. I think the last shot was from behind too, it's not like Flash has super senses.
With that said, I did say the whole fight was dumb and anti-climactic and was practically a throwaway, just saying getting shot by an arrow unaware in the back is not the stupidest or most ridiculous thing to happen in the show, it's actually plausible.
Arrows only travel up around 200 MPH.
Barry was able to catch a bullet he was unaware of fighting that teleport chick about 10 episodes ago.
Bullets travel near the speed of sound (depending on ammo type and gun used).
Barry can catch bullets he's unaware of yet Wells, who is multiple times over faster than Barry, cant catch a damn arrow? Not plausible, just horrible writing for these pointless crossovers.