Am I supposed to trust them in that period?

All you can do is believe them and wait. If you still hear nothing, consider involving the USCIS ombudsmen if the processing time has passed long ago. Is this delay due to document inefficiencies?

What type of case is this and what does the online case status (if available) indicate? If this is an I-751 (the processing time sounds about right), there may not be anything online for the case number listed on the initial receipt notice; you will need to try the number that appears on the biometrics appointment notice when checking your case status online at https://egov.uscis.gov/casestatus/landing.do.