Do these airline companies offer any refund policies for when the ticket price decreases after I've made my reservation?

You should probably check with the companies themselves about specific refunds if the price drops after you book, since I can't answer for every single airline. That being said, here are a few general things to keep in mind with refunds such as these.

1. From experience, I can almost guarantee that Ryanair doesn't offer such a program. Ryanair rarely offers any frills like this. Whether or not you consider it a frill, rest assured, Ryanair does, and in keeping with their no-frills policy, they likely won't offer it.

2. Many airlines will treat such a refund like a reservation/booking change and will charge you a fee for it. Depending on this fee, you might be better off simply ignoring the price change. I recall one airline in Europe (Al Italia, perhaps?) that offered to refund me the price difference of €25 if I paid a change fee of €75. Obviously, this wasn't worth it. In the US, these fees can sometimes run over $100-$150, so the price differential needs to be fairly large to make the refund beneficial.

3. I don't know why Yapta/TripIt hasn't notified you of these changes. Once again, check with them; they may have policies where they don't notify you unless the price change is above a certain amount, or it could be a bug in their system, or it could be a hundred different things.

4. Also, many airlines won't allow you to claim these refunds if you booked your ticket through a third-party site like Skyscanner. Some of these third-party sites will offer their own discount programs, but others won't. It's safe to assume that if you buy your ticket through Skyscanner or another similar site, registering your reservation details through Yapta/TripIt won't help since the airline won't refund the difference regardless.

5. Some airlines will only refund the difference in the form of a time-limited travel voucher for future flights with the same airline. This isn't a problem if you fly frequently, but if you don't, the voucher may expire before you fly again.

Source: I mostly know the above from time I spent living in Europe, but I managed to find a two articles (1 and 2 that speak to points 2 and 5. They're specific to the US, but I've experienced similar situations with many European airlines.