Hello!
...its now more than one months ago ...
One month? That's the time you have to wait for your regular salary as an employee. Why would they use freelancers, if they paid them like employees ?
But seriously: Did you state a date on your invoice by which the amount is due? If not, the legal rules of your country apply. Best thing is to call their office and politely ask, when you might expect your money. If you don't get it by the announced date, a quick visit to a lawyer of your choice might be a good idea.
Over the years, as a freelance pilot (and aerospace engineer) I have experienced payment "delays" from three days to three months. And occasionally "never" when the company went bust in the meantime. Interestingly, there are very large super-wealthy intercontinental industrial firms that delay every payment to the last moment that is legally possible (+/- 3 months). And there are small family businesses (like the flying school for which I instruct) that pay your bills the same day they have it in the mail...
Good luck,
Max