Just a week ago, I built 2 small closed terrariums comprised of a variety of 3 plants each (I unfortunately do not know the species of these plants as they were on sale and simply listed as tropical in the terrarium section) and while one of the terrariums has been seemingly thriving so far, the other one has been a disaster. Just a few days ago, it seemed to be doing fine, but after looking at it today, I noticed the following:

1. two of the plants was starting to brown, one of which was most likely dying

2. I could see small black flying insects in the terrarium

3. there were white web-like strands all over the terrarium

What I am wondering is why this terrarium ended up like this, while the other terrarium did not. Could this have to do with humidity? I will say there is quite a bit of condensation forming, but the thing is, there is quite a bit of condensation forming on the thriving terrarium as well... Any ideas?