Hi Maricarmen,
I hope you have maintained wild card char * in CRMRFCPAR table in CRM as well as in ERP.Please check and confirm and also check in ERP side what is the value maintain in SMOFPARSFA for max running object . generally it is 5.
If all fine will request you to check What is the current replication status in SMOFDSTATE table for running status. in case you are able to see running request in this table and not in R3AR3 or in R3AM1 will request please remove the entries in debugging you will than able to run the request download.I hope it will help you.
Thanks,
Prem