Schweta, I have been on UW for almost three years, and the frequency of getting invitations ebbs and flows more or less in keeping with the factors Martina and Petra have pointed out.
When I am busy (I work both on and off UW), I don't send proposals here and I start receiving fewer invitations. When I need more work, I send more proposals. Around the same time that I get busy again, I start getting more invitations (which I then have to turn down).
I agree with others that you gotta go out and bid in the open marketplace a bit to see invitations. I'm getting less invitations these days but they are also better targeted when I do get them so that's kinda nice.
The last month or so, Upwork have been limiting the number of invitations a client can make to a job post. I can literally invite only 3 -5 contractors to a job post now unless I pay a large fee to upwork