80 Is the number of participants who have been active in the past 90 days so, as you say it is very strict or narrow, and they may only drop in once every couple of months but since you have only had 9, I guess that some have not been taking you up on the offer.
That could be because the hourly rate is not all that much to your demographic if you were trying to recruit professionals for instance. in that case increasing payment may work.
You could also include in the description the meaning of the research if that is possible prior to the study, so that people feel motivated to participate for the public good.
You might also include in the description any benefits that the participants might gain, such as fun (if it is fun), or insights about themselves, and possibly the offer of the results about how they compare to others.
Another issue is that 45 minutes is very long and I think that a lot of participants do Prolific studies in the gaps of their day. If there were any way of breaking the study up into study 1, study 2 and and study 3 of 15 minutes each you may get more of your target group to respond. If it is essential that participants take all three then you could give bonuses, or gradually up the payment.
Or, of course, you could reduce the strictness of the demographic. I wonder why it is so narrow, and if they are busy, or in any way challenged (managers, people with lots of children, disabilities, etc.) people.
As someone recently mentioned regarding ‘people trying to give up smoking,’ there may in a sense be more of those people than the number of those participants who answered ‘yes I am trying to give up smoking’ to the Prolific prescreener because they might be trying a bit (I think that all the time I was a smoker, I was always trying a bit to give up but not enough to be able to announce “I am a smoker who is in the process of giving up”). So you could try your own low paying prescreening survey to try and catch those sort of people and encourage them to take the survey, either by asking them to change their “about you” information (which is non optimal), or better by whitelisting them on a duplicate study which has the relevant prescreener removed.
E.g. If you are looking for people with subordinates (I have answered no, I think) then you might do a prescreener to catch grey-zone subordinate-possessors with a question like “do you ever give requests to fellow workers for work for them to perform” (to which I would answer yes), then remove the “has subordinates” prescreener, but white list only those that have ‘power of request over coworkers.’
I have also recommended in the past that those seeking a small demographic advertise outside of Prolific such as via linked in or Facebook groups. If the people you are seeking are not all that rich then they might be persuaded to sign up with Prolific.
Sorry I can’t be of more help.