The beauty of being grabbed is that you can feel exactly where your partner's hand or hands are, and with a little sensitivity the rest of his or her body too. If (s)he moves you can sense that as well.
On the downside, you are now under attack, so distract! Strikes and kiais are the obvious way to go, but off-balancing and mis-aligning can help too.
Now: In every class of any size there will be people who are strong enough and non-cooperative enough (at least when starting out) to prevent some of the basic leverage tricks of jiu-jitsu from working in their unadorned form. They know what to expect in terms of the specific escape being attempted -- it's a pre-arranged exercise! -- and use this knowledge plus their strength to foil the technique. They are in fact cheating. So it's reasonable to tell them to upbraid them, but there is an alternative: Distract! Take their mind somewhere else. Strike and off-balance, and -- if necessary -- strike again! Even threatening a vulnerable area (please don't make actual contact in training) should be enough to weaken the grip.
If you are skilled, you can use their resistance to flow into something else, ideally making use of their force. But early on, to keep it simple, distract!
After a while everyone should learn that when practicing this kind of self-defence that the name-of-the-game as the attacker is to provide appropriate levels of force to allow the defender to refine his or her technique. It's the Goldilocks principle: Not too little, not too much, just right!
Once the technique is technically excellent and second nature for the defender, it is fun to play with high levels of resistance, but to be realistic this should be during practice where the attacker does not know what the defender's response will be. Don't try to skip to this stage straight away!
That's a key difference between learning or refining a technique vs. testing out your self-defence capabilities under a bit of stress.