First of all, unofficial survey suggests that only fraction of appointments turn into reviews. In asking various providers over the years, I was given data somewhere from 10-20%. That's 1 of 2 out of 10. The gals that I talked to DO NOT request to be reviewed.  

That being said, following are some of the variables that can affect how often someone gets reviewed

Sometimes providers only play with select group of guys that they have seen before, and in those cases there will be no new reviews.  

Some gals do not want to be reviewed anymore, and make sure they will only see those guys who will comply with their expectations.  

Some may have taken a break from escorting.  

Some may have gone UTR. On TER, at least, you need online presence to get reviewed. and if you have a password protected website, you better have some ADs outside of that to get reviewed

and many gals don't want it.  If she's seeing regulars, she might not get reviews.  She also might have taken time off for various reasons...  work, school, relationships, legal issues, child custody.  A lot can happen in a year.  
Not a Red Flag for me but maybe a Yellow.  I think checking her ads...  sometimes they have an origin date which would show they've not been deleted & are not new as someone returning to the business.  I think it's fair to gently ask if she has any recent reviews since the only ones you see are over a year old.  Keeping in mind, no discussion of sex or payment...

No...UTR!

I prefer catering to gents who do not want to share.  lol

I see mostly regulars, and this site is not very popular in my area.

Yes, there could be red flag reasons for lags, or it could simply be other reasons.  To find out you might try pm'ing a few of the reviewers to see if they have recent info on the lady.  If it's a regular, they will.  ; )