You're using an outdated version of GNU Radio, and within that, you're using the explicitly deprecated Packet Encoder, which randomly drops data. Don't do that, you had to pick that block from a category called "Deprecated" for a reason.
Also, it looks like you're using Throttle together with hardware. That doesn't work, and GRC also explicitly tells you it's a bad idea – you should pay attention to the console output in the lower left, it contains useful information!
So, don't start a new project on GNU Radio 3.7; GNU Radio 3.8 or 3.9 is trivial to install from Linux distro sources on any modern Linux Distro, and on Windows, you can use the excellent anaconda installer method.
Afterwards, orient on the examples that come with GNU Radio (installed, typically, under /usr/share/gnuradio/examples) in the digital subfolder. packet_loopback and other files will be good examples to learn from!
Other than that, make sure you've clicked on the "Get Started" button on gnuradio.org, and followed the tutorials up to the point of the PSK transmission tutorial. This should be a good entry to exactly what you're doing.
