Hi,
Do you have a Sequence command containing randomize("experiment")? From what you describe, I suspect that there’s a typo in one of the rows’ Filename column, which results in your experiment failing to preload that resource, and that the randomize function shows the corresponding trial sometimes at the beginning of the experiment, sometimes later in the sequence of trials. Make sure you double-check Filename for each row in your table, and each filename in your project’s Resources folder
Jeremy