It seems to me one of the main determining factors is land mass v population. I guess its fairly obvious that countries with high density populations will fare better in producing a find than those with low density populations. The latter indicative of countries with large uninhabited areas.
As an example:
Disappeared Child Country Density Population KM2 Discovered Rank1. Beaumont children Aus 3 N 192
2. Azaria Chamberlin Aus 3 N 192
3. Keith Bennett Eng 274 N 32
4. Sarah Payne Eng 274 Y 32
5. Karen Hadaway Eng 274 Y 32
6. Nicola Fellows Eng 274 Y 32
9. Madeleine McCann Por 112 N 73
8. Ben Needham Kos 81 N 97
9. Jennifer Klein US 34 N 146
10. Joanna Cipriano POR 112 N 73
11. Rui Pedro POR 112 N 73
12. Rene Hasee POR 112 N 73
NOTES:
1. Keith Bennett is an anomaly but we know his body is undiscovered on The Moors
3, 4, 5 children disappeared and discovered within England but figs are for whole of UK
8. disappeared from the Greek island of Kos but figs are for whole of Greece
https://en.wikipedia.org/wiki/List_of_countries_and_dependencies_by_population_density