issue with loading text data to igor

Hi,
i have exported a single channel trace data from patchmaster in igor pro format(as text data), and then import the text data by igor, and the funny thing is sometimes i can successfully load the trace, some times it just show some backgroud( i mean some X Y axis), no trace, i tried many time, i found if the trace i choose from patchmaster is at the beginning recording( i mean less than 100 seconds), the trace show up, other wise it dont show up.
when i export the trace from patchmaster i choose igor pro format, allow raw data access, trace relative to sweep.
Can someone help me fix the problem?
thank you very much!