In case you haven't noticed, Google has updated their Google Opinions (Rewards) app in order to make it more GDPR compliant. The new setting is tucked away in the settings panel on the left — tap the hamburger icon, hit Settings and then you'll be able to make some changes from there.
If you've also moved to a new country, or your age has changed, or you've decided that you want to be identified as a different gender, you can do that too in the same setting panel. These changes will have an impact on what sort of surveys you'll get and consequently the payout per each survey; it's likely that being identified as female might actually land you more surveys, although this is just merely speculation on my part.
If you suspect that your account is broken because you haven't been receiving surveys in a long time, completely nuking your Rewards profile and then starting all over again might be a good way to 'reset' and fix any problems that you previously might've had with Opinions.
As a side note, does the credit ever actually expire? I've bought all the apps I've ever wanted (I don't play any games on my phone) so I have $15 just sitting around. I could use it for a subscription service, but there aren't any I'm interested in.