Clotting of fibers in the dialyzer usually indicates that not enough heparin was used during the tx. Or, if heparin is not advised, that the dialyzer was not regularly flushed with saline during dialysis. Often, it is not a problem (such as in an acute setting where reuse is not practiced).
In general, however, you would want to increase the pt's routine heparin dose. Your clinic may have protocols by how much the RN can adjust the dose; or, you may need to call the nephrologist. In the clinic where I worked, we would increase the loading and/or maintenance (hourly) heparin dose per protocol, but at the RN's discretion (e.g., from 2000U to 4000U); the nephrologist would then sign off on the order.
Your manager or charge nurse should give you guidance. Especially in a reuse unit, they would be happy to hear that you noticed this.