Just to ensure we have not skipped any first level questions .... I've done that too often in the past.
In addition to Olly's question, is your calculated field large enough for the combined fields? Both data and display length? (I'm thinking text fields here - if you are working with numeric ignore this, probably)
If you use the "Show field contents" option you can easily confirm the content of oversize (eg multiline and Memo) fields.