I've noted that whenever I add a new mod to an existing world, sometimes the items and blocks will screw up, and will not appear properly, and in the creative tabs, they will simply appear as other items. I do not have any logs of this at the moment, but, I can see if I can reproduce it.

Your item id's are conflicting.

 

example:

 

Let's say you have an item using the id 1500.

Then you add another mod using that id overriding the id 1500.

your item will change to the other mods item id removing the other item.

 

You should edit the mod configs and check the console for any item id conflicts

I am using 1.7.2 though, which pretty much stops conflicts due to forge gradle.

I think not all mods are using forge gradle then check your config files.

All of you are wrong. 1.7 completely removes block/itemIDs for modders. Modders only need to interact with their block object.

 

OP, I need a screenshot.