I am struggling with making a cloud chamber myself. But the essence of a cloud chamber is a temperature difference. This has to do with the fact that the amount of moisture air can contain is dependent on the temperature. The top should be relatively warm so that the air can contain a lot of evaporated alcohol. And the bottom should be relatively cold so that it cannot hold much alcohol and thus condensates.
The problem in your cloud chamber could be that just the bottom is really cold but the top is not warm enough. This way the whole chamber just gets really cold and then there is no temperature difference. This could also be an explanation of why it only works in the first few seconds. Becasue at first the temperature of the top of the/whole chamber is room temperatuur. But then it quickly cools down. I assume that the big bowl on top is where you put warm water in? It might be that the plastic is not conducting the warmth enough to the filt. Plastic is not a good warmth conductor. You first have the plastic bottom of the bowl and then the plastic of the top of the chamber. That are two layers of plastic the warmth need to go through.
As a possibe fix I would try to put a metal bowl of hot water on top (for example a big frypan). Besides that, the filt really needs to be soaked with alcohol. (First get rid of the surplus alcohol before you put it on the dry ice). I also recommend using a really pure isopropanol (I have 99.9%). I would also recommend using a black (metal for conductivity) bottom plate, because it makes the trails more visible.