The problem is that Cheat Engine 7.1 (can't speak for earlier versions but they're likely the same) has an issue that causes it to miss some values while scanning. Halo's plasma ammo seems to be the most effected by this. Cheat Engine 7.2 will include a patch that fixes this issue, and an early release of this version is on CE's patreon page.
I'll quote Dark Byte for anyone who wishes to know more details on what's causing the issue.
If anyone wishes, I can help to find simple values for Halo 4 before my table's release, just don't expect me to find something difficult to find.There is a bug in CE's scan routine that causes it to fail on 2GB+ memoryregions when doing an unknown initial value scan. (Signed size variable)
Normally games don't allocate chunks of memory in that size, but some emulators, and ported games (that likely have an emulator built-in) do, in which case when you do an unknown initial value scan, it won't find the value
For games with this issue, it is possible to bypass this issue if you can find some common code effecting a value you can find and a value you can't find. A good example would be recoil or rapid fire for weapons.
EDIT: Now that Cheat Engine 7.2 is officially released this is no longer an issue.