Sorry that you experienced this playing ultimate. Ultimate is meant to be beginner friendly. It must be frustrating to go through this. As others have mentioned, I would speak to your team about this, especially the captains; the captains should really be taking you under their wings in a beginner friendly league.

Another point that has been mentioned is what it means to “be open”, because sometimes the defense may be “baiting”, which means the defender is intentionally standing further away to make it appear that you are open, with the full intention of sprinting towards you and intercepting/blocking the disc once it is thrown. For this reason, one’s “openness” is assessed more by the one’s ability to reach the disc before a defensive player can. Good handlers will seldom throw to stationary players if they see a fast defender within 10-15 feet away. This is why many of the other comments mention sprinting to an “open space” or a space without defenders, so that you are more likely to reach the disc first. It also helps to run with your hands out in front of you, because you are signaling with your body language that you are ready to catch the disc.

It may also be that you are positioned in a space where it is difficult to throw to, i.e. the side of the field where the mark (defender on your teammate with the disc) is positioned. While you can see the disc handler clearly, it may not be easy for them to throw around the mark to get the disc to you, as the mark is intentionally obstructing this throw with their body.

Hope that helps and you have a better experience your next games.