Oh, I think it's because you forgot to end the first tag before starting a new one. You have a "{bt=30}text {bt=30}" When you should do a "{bt=30}text{/bt}" The `{/ bt}` Will end the previous instance of the tag and doesn't require the =30 or whatever to signal it. But with all my text tags, you must include the {/} at some point.
Least that's what I think it happening. If I saw your script I'd probably have a better idea on what's going wrong.