I was hoping from a few clarifications from the OP but I'd like to start addressing a few points related to some concerns.
As @user71659 pointed out and like I hinted at in my question/comment the credit bureaus don't keep scores and so there is no history they can provide (heck, no present score too). There are some standard scoring models, some used more than others (FICO 9, FICO 5, VantageScore are a few that come to mind and there may be a comprehensive list somewhere online).
If you haven't already been doing, you should use:
- Credit Karma (unless you have reservations against signing up for services that may potentially use your data) or some such app/site. This gives you monthly scores based on a scoring model. So this may not be very accurate and it may lag by a month or 2 at times depending on how frequently Credit Karma polls the card/loan provider and how often they get the data refresh.
- Credit monitoring services that (these days) a lot of credit card providers provide for free. Again, these would be based on one model and most of them seem to poll Trans Union records. So if you have issues from Experian and Equifax, then these may not help. Credit Karma at least gives you tracking from TU and Equifax.
If your wife never missed a payment by 30 days or less (i.e. always paid at least min balance on time) the scores may dip for a month or two because of the refresh rate by the polling services (explained above). However, it should not be permanent. Your OP seems to note that that dip has persisted for more than 6 months now. That can only be because of a missed payment and other factors. So 2 cases:
- If there was never a missed payment, request reports from all 3 bureaus for free (once a year for free) and then inspect every single item to see if there is a mistake and then call the bureaus to report the mistakes. This may need follow-ups from the loan/credit providers confirming the mistake.
- If there was something wrong then the bureaus won't do anything. However, you could call the creditor and demonstrate that everything was eventually paid off and see if they could mark the late payment as on-time payment. This is possible and usually credit cards are lenient in reporting late payments unless it was more than 60 days or 90 days. Once this is done, you could call the bureau to confirm this.
All said and done, the scoring models differ and when you apply to new loans/mortgage they may or may not use a particular model but multiple and also look into reports for things that stand out like late payments, collections, avg credit utilization, income growth etc. So putting aside the fluctuations as normal there still is an indicator that there may be a mistake that crept in.