There can be two answers, here.
First the easy one: the price you will pay is the price that you agreed at the moment of booking, so expect no raises and no discounts.
Then, the complex answer to "Is there a specific time I should pay for the hotels?"
What you are observing is the standard way it works with hotels, and it's the total opposite of what happens with planes.
Premise: for ease of explanation let's fix a day, let say you'll go on holiday the 15th of March.
Airlines make money with last "minutes" bookings. They know that those that will book just a few days before or even the same day of the flight, maybe at the airport counter, are those who need to fly; often are business travelers which have no problems of money at all. Thus, airplane's fares are very cheap when booking far from the 15th of March, and increase as you approach it. In a sense, it's actually those who are booking between 12th and 15th of March who are paying the flight for everybody else.
Hotels, on the other hand, work exactly the opposite. A hotel has no expenses and has no "last minute business customers", and for a hotel an empty room is a 100% missed gain. And they know that peoples (the same who book a flight months or an year in advance) tend to plan their holidays quite early, are willing to pay more to stay in the hotel they have chosen, and to avoid the irrational fear (more on this later) to find no place to stay. So a hotel has a fixed fare for 15th of March that is quite high, and that everybody will be happy to pay to secure a room.
As time passes, anyway, the hotel needs to be sure to fill all of its rooms: as I said, an empty room is a 100% missed gain. So fares tend to drop (a lot) the more you approach the 15th of March, and if you book the previous day you will find at the hotel a few discounts.
BUT...then there are sites like Booking.com. Those sites provide free advertising to the hotel and allow them to reach a huge audience that would be really expensive to reach on their own, so the hotel has all the interests to sell some of its room through those sites. And this means that the hotel allow booking sites to drop prices a lot. After all, as we said, an empty room etc. etc.
In the end this means two things:
Yes, what you see is perfectly normal. The hotels you booked will likely drop in price down to an 80% as your holiday will approach. But you will obviously pay what you agreed at the moment of booking, no discounts
Unless it's a very busy place and in high season, or you are easy to panic, or you just want that exact hotel, it's better to book (and pay) at the last minute.
Just for reference, in Dubai I spent 42 euro a night for a suite that is normally 350 euros (the power of overbooking, too), and did more or less the same in Indonesia, just booking when in front of hotels and getting discounts from 40% to 70%. And so on, and on...