This is some nice thumbnailing. There are some cool designs in there but it lacks some direction.
Things i'd like to mention:
Material: it is hard to tell what each material is, my guess would be its dirty rusty metal, but it could also be just mud. Try to find a material you want to go for. Rust is something thats pretty hard to convey, for me at least, in that resolution. Rust usually works well when accompanied by still clean metal, which makes it difficult to implement in that resolution.
Colors: Colors and materials are kind of related. You went for a lot of earthy tones. There is no color variety, no 'hot spots' to look at, no contrast (neither in color or brightness), everything looks samey and flat. The design seem fun, but they dont look fun.
Design: The individual ideas are pretty cool, but lack flashing out. They have a pretty broken appearance thanks to that, but it doenst seem deliberate, but sloppy. Like atnas said its hard to make out what is what. try to think of the function of the robot and design them accordingly. you can go step by step and just add things here and there, until it is not a bunch of random clusters, but something that feels inherent to the design.
Making them worn out: This can be a tough one. I found it easier to start with the robot in its original state, then continue to a broken one, and finally add some moss and vines. This process can help you to figure out how and why something is broken. The second and third step are actually pretty straight forward once you got the main design down. Just add some scratches and dents, remove some bits, twist some parts and finally add some vines where you'd think they would grow.
Here's a process gif visualizing some of the points i made:

Looking forward to some more bots
