This was certainly a hot topic a number of years ago when dead end ultrafiltration was still a common technique. Sartorius therefore were the first inventors of the vertical and dual vertical membranes used in most centrifugal ultrafiltration devices today. These vertical membranes provided a near parallel flow path across a membrane, meaning there was significantly less fouling and blockages. However if membrane fouling is still being experienced a few suggestions are; implementing of a new or more efficient clarification step, perhaps with a combination of diatomaceous earth and a 0.2um PES membrane filter. Or ultrafiltration through a true cross flow / tangential flow filtration (TFF) device, such as the Vivaflow range for lab use. If membrane fouling is being experienced even with TFF devices and a sturdy clarification process, and as a result a decline in flux is still being seen, it may be worth checking that if a multiuse membrane, the cleaning and storage procedures are being followed correctly. Perhaps using single use devices could resolve this. You could further investigate the membrane properties and test a new membrane to see if the hydrophilicity, charge, binding strength to contaminants, etc, could be having an impact. The two common UF membranes are PES or RC and both can work very well with most samples, but occasionally one is better than the other for a specific sample as ultrafiltration is impacted by both physical and chemical interactions.