Deino Grab?

So at highlands, first time, and I've been stalking that little dammed path at the northend of the lake where frequent crossings occur. Well a Dibble was drinking, I managed to swim up and grab it. But then it fell out of my mouth. Had the stamina to grab two more times, each time lifting it off and having it fully in my jaw, only for it to fall again. I had some spare stamina, and I held RMB while I grabbed. What am I doing wrong? If the dibble would be too big, I shouldn't have been able to get it in my jaw for a few moments, right?