My A2500 came with a 2091 which a not bad controller. It still may be the controller.
But check theses things first:
1- check to make sure the unit is set to a number between "0 and 6" 7 is reserved for the controller
2- try another cable if it is an older cable it can have cracks or breaks
3- check the drive on a known working controller Amiga Windows or Mac based it could be the drive
4- Prob this should be first or secound check to make sure it is a SCSI 1 or SCSI 2 drive (some SCSI 2 drives will not work with SCSI 1 controller) make sure it is not scsi 3, ultra wide scsi, ultra fast scsi or scsi ultra.
If it is the contoller I would suggest a GVP controller over Commodore 2091. It has better addressing and controller software and is between 20-50 % faster and the 8 meg ram update takes standard 30 pin sims.
