@Co

The backgrounds are definitely a big issue. Some of them are patterns/textures, some are gradients, some are muddy soft shading. Most of the time it just looks like these characters are hovering inside of a cardboard box, or something quickly generated by tracing over a photo with the line tool. I think you're trying to hide behind the typical conventions of manga to conceal a particular weakness in your skills here (nothing wrong with that, backgrounds are difficult) but you're not going to grow as an artist if you keep cheating around it.

Your colour choices are a little garish at times, especially with the photo/graphic textures you're using. Some of your textures aren't even properly skewed to be the correct perspective. This can create a weird sense of the scale being off in relation to the characters. 

The characters also all look very similar to me. It looks like the same design used over and over again, but with a wig swap. Even characters that are supposed to be of different genders.

More variety in line weight would also give weight and depth to your images overall. Right now they feel very flat and have a sort of MS Paint quality since everything is the same line size. 

There just seems to be a lack of consistency overall and I think that detracts significantly from the quality of the comic. If you're looking for areas to improve, backgrounds and consistency are a good start. You might also want to get a beta reader who can help you out with your grammar and spelling.