First make sure that the pump itself is working correctly. (The Mitivac pumps like yours are expensive, but also very unreliable.) Disconnect the tubing from your cylinder, and block the end of the tube from the pump with your finger. Now squeeze the pump handle. The pump should very quickly make a strong vacuum, and it should hold. If it does not, there is a leak in the pump itself. If the pump is OK, first make sure the tubing fitting is screwed tight into the cylinder, and you are inserting the hose connection fully when you hook it up. If that's all good, you probably are just not making a good seal between the cylinder and your body, so try trimming the pubes, using various gels or lubricants to get a seal, etc. Also, if the cylinder is too big for you, it may be very hard to get a seal.
However, if you find that there is a leak in the pump itself, make sure that the release valve on the bottom is closing properly. Also make sure the gauge is screwed in tight, as in the video it looks like it is crooked, so it may be loose. If that is OK, put the end of the hose in a glass of water, and suck a little water through the pump. That will sometimes flush out any debris that might have gotten sucked into the pump and is interfering with proper operation.
By the way, if your Mitivac pump has a plastic handle, and you use it a lot, the handle will eventually break. When that happens, do not buy a new Mityvac pump. Go to a Harbor Freight tool store and buy their "Brass Brake Bleeder Pump." It is half the price and much more durable, having completely metal construction. Save the hose and cylinder connector from your old pump, because you will need to attach it to the new one.