I am 6 2 as well. I can't tell you how to do this correctly on aikiweb. Yea you have to bend your knees, but there is a little more to it than this as well.
Grabbling the arm. Probably a big clue as to part of the problem. Grabbing the arm means most likely that you are engaging the arm upon contract before you have established connection and kuzushi.
Grabbing the arm can trigger a proprioceptive response from uke, so if you extend, grab the arm, then bend your knees...well uke will feel the grab, then begin to disengage, bend his knees in order to get lower than you. So most likely he will always be slightly ahead of whatever knee bending you are going to do...thus you will never beat uke to get underneath his center.
Play around with ikkyo some with different things happening at different times.
I personally don't like to grab the arm at all, but to recieve, enter first....uke should not get feedback from the contact and thus he should not have a reason proprioceptively to consolidate and pullback, and bend his knees. Then you can try to "get under" his center, kuzushi established, and uke then needs to find a response to get center back...but it is too late and he is "behind" your action with his...and now you are ahead of his action loop.
Again, I can't coach over aikiweb...but maybe this mental picture might help?
This was a big part of my problem being a big guy and stopping the early grabbing was a huge help for me.