With stories, it is just the same. If your theme is passion, then there must be strong feelings and full hearts. If the theme is jealousy, then green-eyed monsters must be lurking. If the theme is the search for happiness, then there must be a lot of looking, maybe some losing and then some finding.
Notice that the theme can be summarised in just a few words. If you have to ponder for ages when asked about the theme of your latest story, then it isn't going to work. You must have a hook that it all hangs on.
My favourite story, mentioned in a recent post, The Necklace by Maupassant, has the theme of pride coming before a fall. Others might say it is about deception being a tangled web. Or it is about dishonesty. Whichever theme you choose, all of these examples have the same thing in common. They are brief.
Try asking yourself what your rejected stories are about. If you struggle to find the reply or need several sentences to explain it, then you know why thoses stories have not yet been successful. Try inspecting them in detail to winkle out the underlying theme or themes. Perhaps you have too many threads in there? Try teasing some out and tidying the story up until a predominant theme emerges.
Do not confuse theme with plot. More of that in a later post.