The conclusion of Sean Martin's work 'The Cathars, the most Successful Heresy of the Middle Ages'.

...Catharism was, in the years after the fall of Quéribus, a chimerical presence. According to the testimony of Stéphanie de Châteauverdun, a noble and Cathar Perfect from the Sabartès, what top-level Cathars remained were living in the mountains. William Prunel was one such Perfect, whose career stretched from around 1258 until 1283. Despite the tireless efforts of the Inquisition, the one thing that was hardest to eradicate from the Languedoc was the deep roots that Catharism had put down. Even after all the atrocities and hardships that the area had suffered over several decades, people still seemed unwilling to give up completely on the old religion. Evidence to support this comes from the fact that William once spent a month in Toulouse; he was recognised as a Cathar, but no one betrayed him. He continued to spread the faith, and was known to have nobility and clergy amongst his flock. Another Perfect,William Pagès, was also active during the same period, although he had managed to survive by spending time in Lombardy.

Apart from the willingness – or otherwise, as in the case of William Prunel – to betray a known Cathar, the Inquisition faced other problems during this period. Foremost among them was the crucial relationship between Inquisitors, bishops and royal officers. Although the machinery of repression was generally efficient, its effectiveness did vary from area to area. In Narbonne, for instance, hostility towards the Inquisitors had diminished to such an extent by the early 1260s that they were called on to arbitrate on the town’s behalf in a secular dispute with Béziers. In Albi, however, the bishop and the Inquisitors were at loggerheads with royal officials for years over the issue of confiscating the property of convicted Cathars: the bishop favoured leniency to prevent families from being bankrupted, and was, remarkably, supported by the Inquisitors. Royal officials were attacked by crowds of locals; in return, the bishop’s bastides – small fortified new towns – were pillaged by royal forces.

Matters deteriorated during the last two decades of the thirteenth century, with complaints against the Inquisitors rising. The Inquisition hit back, accusing royal officials of complicity with heretics: in the 50 years before 1275, there were only two such complaints, but between 1275 and 1306 there were thirty.
https://exploringrealhistory.blogspot.com/2020/11/part-3-of-3-cathars-most-successful.html