Hi OP, as someone who’s currently going through the process of grading application forms to narrow them down to interview candidates, I’d give the following advice… (Please don’t be offended if you’re already doing all these, it’s just based on common issues across what I’m seeing so far!)
If there are ‘essential skills’ or anything along those lines in the job advert, really signpost good examples for each one in your application. Don’t just say you’re good at something and expect them to take your word for it.
If you can do any research on the company, for example if they have a website that sets out their organisational values, try and get some subtle linkage with your own values, and similar language in there too.
When listing your employment history, do most recent first, and try to align the responsibilities to the role you’re applying for wherever you can.
Get somebody else to check it before you send it, good spelling, grammar and punctuation always give a good impression.
Don’t include any irrelevant information like random hobbies, unless they can help with providing evidence of the skills they want for the job eg if they want teamwork and you play in a sports team or something. Should be additional to the work examples if possible though. If there are elements of your Masters that can be used as evidence do also elaborate on those.
Other than that, keep trying, there’s definitely something out there for you!