Inexpensive vacuum sealer not pulling out air
One of the things I've found critical with getting a vacuum using a fairly similar style unit is to make sure the air has a way to escape from the bag reliably. The best bags I've found are ones that are smooth on one side and have a cross-hatch ribbing on the other side, I've had limited success with bags that are smooth on both sides that presumably are OK with commercial machines.

Apart from that it seems important to cut the bags fairly straight if not using pre-formed ones and to make sure the end of the bag is in the center of the channel right the way along. I've also found it's worth trying to keep the end of the bag slightly open so don't try and push it down just let it sit there and rely on pressure from the lid to hold it down while operating it. You also seem to get best results with a nice even pressure on the lid, I use my palms rather than fingers to press down and keep them towards but not right on the end of the lid.