thought i'd post this in order to save someone some time.
when importing certain ofx files (from certain banks), the register comes up blank. in my case, the problem was the FITID field of the file. apparently, from my bank this field does not get assigned a unique value, so gnucash thinks that it's a duplicate transaction and doesn't import it.
solutions:
1. quick n dirty: find/replace the FITID value with a unique value. $DATE1, 2, 3, etc. this has problems if you download the same transaction more than once (overlap the dates, etc)
2. better would be to replace the FITID value with the datestamp of the transaction. i'm sure it would be trivial to write a script to do this, but i don't know how.