Building on
@Bert Preast's comments:
- Traps generally need appropriate bait (for what you're trying to catch), although trap lines can catch small game if they're long/numerous.
- As mentioned, it's very uneven, so it can go a month without you even seeing a bird, and then suddenly you catch a lot of them over a short period of time.
- Traps have been reworked comparatively recently. One of the changes I've seen is an increased chance that a trapped animal manages to free itself and escape, in particular if you're able to see the animal getting into the trap. The chance of finding a triggered but empty trap when visiting traps is a lot lower. For that reason I have my character face away from the homestead small game traps while performing activities.
- Another change is that I've seen birds pass through/over small game traps without triggering them.
- Another change is that large game now ignore and trigger small game traps (i.e. just runs straight through them).
- I've also had recent issues with an elk running straight through a bear trap without triggering it, both while I was passing to the other side of the trap fence via the overmap (it was on the other side when I moved, but had moved to the starting side when I "landed"), as well as while chasing it. This has led me to question the effectiveness of bear traps against large herbivorous game, which is a real concern for my current character, both because it's winter so no digging can be done, and the whole region is mire, so there's nowhere I can dig a pit trap.
- It does seem that you need to refresh the wildlife near your traps. I try to visit the traps once every day on a short tour, which can cause you to (occasionally) find animals in them when you return. I disable traps further away when I don't want to spend the effort to check them as a role playing measure: capturing animals and then not collecting them is "wrong", even if nothing is actually generated in the area by the game (but you can never know if something was generated on your last visit and then entered the trap while/shortly after you'd left).
- When it comes to the smallest lever traps, I place them in lines (i.e. one next to the other) as some kind of trap fence to block the passage of birds, hares, and foxes to increase the change of something being trapped even when they're not baited. That's crucial in the early game when there's nothing to bait traps with.
- You can use "natural" bait in the form of berry shrub by placing traps on them hopefully luring berry eating animals into those traps. You can also pick berries and bait traps with them, and you can also bait traps with e.g. fat (using bark for tanning instead) to try to catch carnivores. Most of the time bait spoils before anything is caught, though.
- Trapping skill probably plays a role. However, my characters are usually poor at trapping, and it's a skill that doesn't increase much through normal use (i.e. to become good at it you probably have to perform dedicated training by setting, triggering and resetting traps repeatedly).
- One method that can be used is to set up a baited trap in an area where you've seen an animal. That sometimes work. Note that using that method to try to trap bears and wolves may result in you getting attacked while working on the trap or checking it. That's a particular danger with wolves, as your trap may catch one wolf, but the rest of the pack is then ready to greet you (lost my last character to this when the pack was far too close to the homestead so I had to take some action)...