There are three issues you may be running into: Sort order issues and "smart search" features that control automatic category navigation and keyword substitution, and the fact that eBay considers title and item-specifics when deciding if a listing matches your search.
Using an exclusion term (a keyword with a minus sign in front of it) or using quotes around a keyword will bypass any smart search features and force eBay to return only exact matches to your keywords.
The "Price plus shipping: lowest first" sort order provides filtered results -- often drastically filtered. And the option to "see more results" may not always be shown as it should. The filtering may vary depending on the keywords used.
If you encounter a drastically filtered result for a keyword search using a desktop browser, you should be able to use the "Feedback" link in the lower right corner of the search results to provide feedback on your search to eBay developers. The more users provide info, the more likely something is to be done about the over-filtration problem.
You can avoid that problem entirely by using a different sort order.
If you have few results, try sorting by "highest first", then starting at the end of the results, scroll up instead of down. This also avoids the problem of variation listings with low-cost variations appearing at the top of the lowest-first sort.
If you have a bookmarked or saved lowest-first search that you use a lot, you can avoid the filtration by adding the following to the end of the search URL, then saving the resulting search:
&_blrs=recall_filtering
"Best Match" sort order seems, lately, to be providing more results than other sort orders so that can also be an issue when searching and changing sort orders.
