Actually ads are not the only ones that get burned out. Landing pages get burned out too.
It may happen that visitors click the ad by mistake or just out of curiosity then land to a landing page they’ve seen quite a lot of time.
I would suggest trying to add other landing pages or at least other variations of the same landing page. Sometimes even a color change in the theme of the lander can help on boosting the results.
What about quality of traffic? I assume the longer you run, the more traffic sources "need" to fill your budgets with traffic. This could be any type of low quality traffic (bots, people who engage/convert lower than others, etc), hence the lower CTR and CR eventually. Just a thought...
I would say it all has to do with the traffic distribution. It's quite typical for CPC sources... I assume you're buying based on the CPC model, right?
The source wants to make the most money, so they push the creatives with the highest CTRs (combined with the bid of course) the most. So as your AD ctr goes down, the algo gives you less and less exposure and you're losing one good zone after another, plus you're being pushed down in the bidding chain.
Usually, the best traffic comes from the early ad impressions, so the deeper you are in the bidding chain, the lower quality traffic you are getting... which results in lower LP ctrs and lower conversions.
In order to stay competitive and get as much of the good clicks as possible, you need to refresh your ADs often in order to stay high in the bidding chain.