Just went into game (with most update game and mod versions) and tested putting into ammo, everything is alright. So something must be wrong in your game. Can you check the following?
1. The caliber (88 mm, 128 mm) of cannon and shell must match. It is denoted by the little blue dot at upper right corner of the icon (both cannon and shell). Heavy tank using 88 mm cannon (1 dot) can only use 88 mm shell (1 dot). Super heavy tank uses 128 mm (2 dots).
2. If the insert of shell fails, there should be an error message on lower left corner of your screen. Can you show me the message? (Either text or screenshot.)
3. Can you post a screenshot of when you trying to insert the shell? (Including your mouse cursor holding the shell item, and the cannon slot of the tank.)
4. Do you have other mods installed? Especially those may affect weaponry?
5. Have you tried disabling all other mods (just with Schall Tank Platoon), and try placing the shells again?
6. If all of the above fail, can you provide your full mod list and/or savegame?