Kneebars on guys with long legs

Sometimes when I roll around I have trouble finishing kneebars against guys who are rather tall and large (6'3" to 6'5" weighing anywhere from 220 to 280lbs) as I am just about average in heigh and size (5'9" and about 200lbs). Does anyone else run into this problem?

Granted I am a newbie to the submission game, but I think that perhaps it has something to do with the leverage advantage that they have because their legs are long....am I correct?

If not, could you guys give me some possible suggestions about what I could be doing wrong (I understand that the only way to accurately determine what I am doing wrong is to actually observe me, so I am not asking for you guys to give me a definitive answer, only possible suggestions based off your knowledge and experience), and what I should be doing right in order to pull off the kneebar against guys that large?

its hard leg locking anyone that size, go for other things, take their back or something, something that is harder for them to deal w cause they are bigger.
-Simmler

ditto.

people that big are hard to leg lock (kneebar). Since
they outsize you by a lot, that huge ass muscle can
defend pretty easily against your whole body. Go for
chokes, arm locks, neck cranks, or even ankle locks.
But don't go for the kneebar, unless you are really
good and can catch them with their leg extended.

Sometimes I have trouble finishing kneebars on people
my size or slightly bigger. It's easiest against
people you are bigger than.

Some tips on finishing them though: try to attack the
leg quickly and get it extended as soon as possible.
Once you get it extended, they are finished. Keep
their toes pointing towards your chest and straight
through your back. Isolate the leg with your two
legs, squeeze your knees together. Switch to a toe
hold if fails.

good luck bro.

If the leg is too long to submit by hugging it to your chest (like an armbar), try passing it under your armpit, like you're guillotine choking his ankle. Works really well, in fact I finish most knee bars like this now.