The issue with the checkBalance function not appearing in the Candid UI or responding to CLI calls is likely due to its incorrect declaration using async in a shared query function, which isn't necessary and can cause it to be excluded from the generated interface. To fix this, change the method signature to public shared query func checkBalance(): Nat (removing async), then stop the DFX server, delete the .dfx folder, and restart everything cleanly using dfx stop, rm -rf .dfx, dfx start --clean, and dfx deploy. After redeploying, the method should appear in the Candid UI and respond properly to dfx canister call commands.