Your .cvs file does not reveal any obvious causes of this unintended skip. Have you looked carefully at your IAPS_rand file? How many items are in that file? Are there errors or formatting irregularities that might lead to it being read as only having 4 items?

Since your .cvs file does not show any explicit command to skip, I would think something in your IAPS_rand file is indicating you're done after four trials; almost as if it thought you only had 4 items to choose from. But you have 20, correct?

Attaching that file would be helpful in replicating the error.