You cannot ask the buyer to pay more, unless you want a very annoyed buyer and most likely a red x for feedback

"Automatic Offers" are often the answer  to your question about selling less than the advertised price.

Sellers forget or don't realise they're enabled.

Scroll down to  "Automatically Send Offers"  in this link and it will tell you how to disable if  necessary:

https://www.ebay.com.au/help/selling/listings/selling-buy-now/adding-best-offer-listing?id=4144&st=3&pos=2&query=Adding%20Best%20Offer%20to%20your%20listing%20or%20using%20Reply%20with%20offer&intent=oiffers&lucenceai=lucenceai&docId=HELP1043#section5

Did you have offers enabled? Did you have auto-acceptance of offers enabled?

That is the likely reason for such things to happen. eBay generally don't arbitrarily change seller prices.

I've heard allegations of same, but in 17 years have never experienced it.

Check your settings. Ensure you don't accept offers or, if you do, make sure you have a minimum for auto-accept.

If it's something else a bit more detail will be required for other members to provide cogent advice.