Sshifrin, it may be because you are too slow to sit up. When doing both of these escapes, it is critical to post your arm or elbow on the mat and get your head up as soon as possible. Personally, I try to post my elbow on the mat the exact same time my other hand completes its elbow push towards my hips (this requires you to raise your legs in advance of the elbow push). Also, getting your head up is important; you basically need to do a crunch to get it as high off the ground as possible, which is feasible since their weight is off your chest after the elbow push. Note that this escape requires energy, speed, and core abdominal muscle activation, but that is true of every sidemount escape (as the comments note, all sidemount escapes consume energy, so better to avoid getting your guard passed!).

Once your elbow is on the mat and your head is up, if they try to go to north-south you can turtle with percentage, and if you may be able to reverse them since they will need to raise their hips to get over your head. Once your hand is on the mat, they will probably not even be able to get to north-south since they can't get their hips over your head, and if they do manage to get them that high you will have a high chance of reversing them. If you watch Marcelo spar, you can see him reverse several of his opponents as they try to go to north-south.

One last detail that may help is elbow / hand placement. It's important to try to place your arm on the mat slightly behind and to the left of your head so that you will have good leverage to resist their north-south movement. If your arm is posted too close to your hips, you probably will not have a good angle to resist his motion and you will get squashed. By way of example, if you pause at 2:47, where is elbow is on the mat, you can see it is placed behind his head (and his head is already up). At 2:49, where his elbow is on the mat, you can see it is placed even further behind his head (and his head is even higher up).

Hope that helps!