Can't put grenade on Scorpion or Wraith?

[EliteSniper]

I was playing BTB earlier, and once I boarded an enemy Scorpion, I didn't have the option to put a grenade into the tank even I had 2 frags. Same thing happened to a Wraith, I could only punch it, and it takes a looooong time to destroy it ( btw I got sniped -.-)

Then, I played campaign to test it out, but I was able to put a grenade on a Wraith.

 

NOTE: I boarded the Scorpion from the back side and the Wraith from the front, is that the matter?

I'm really confused

[EliteSniper]

Lol why did I got a like?!?! O.o