The guard break you are describing has two key components - pinning your opponent's hips to the ground and opening your hips wide after you've stuck your knee against their butt.
Assuming you're doing this correctly pushing on the inside of the knee will add additional pressure but only after you've done the above or you're simply pushing their leg into your own thigh. The timing on this is important: too early and you're simply wasting energy.
If it's still not working - and sometimes it won't if your opponent is tall or strong enough - you're going to have try a different guard break. Standing in some fashion is usually required to break this kind of closed guard if the guard player won't open their legs to attack.
Hope this helps.