I'm not sure I would say 'forgot' as that sounds quite accidental to me, rather I would say 'abandoned.'
So yes, Polynesians definitely did have a pottery tradition in the beginning. The Lapita peoples who first settled the central Pacific archipelagos of Fiji-Tonga-Samoa sometime before 1000 BCE brought a distinctive pottery style with them. The descendants of the Lapita developed into what we now sometimes label as Polynesian cultures. Indeed, if anyone is familiar with the Lapita culture, it is often due to their distinctive stamped pottery style. Lapita peoples used clay pottery for a variety of purposes; storing goods, cooking, and as prestige objects. Individual pieces of pottery were treated as prestige objects that were collected, traded, and circulated in the normal course of life.
By about 300 CE the usage of pottery declines in the Central Pacific and certainly by 1000 CE it was abandoned (with the exception of Fiji). This appears to be the end result of a long term shift in the way people thought about and used pottery. When Lapita long distance exchange networks begin to decline around 500 BCE and cultural divergence really sped up between settlements in the Western and Central Pacific, the usage of traditional Lapita pottery changed. The first major change was its role as a prestige good-- indeed the pottery in the Central Pacific underwent a slow simplification. The stamped patterns and designs, which were stylistically very important among the Lapita, simplified and then totally disappeared. At the same time, those patterns and methods of visual representation shifted away from pottery and toward tattooing. While we don't have records of tattooing from that long ago, it is clear that at some point the visual language of the Lapita pottery shifted to a new format and has continued and continued to develop up through the present day.
The simplified pottery that developed in the Central Pacific shifts from being labeled by archeologists as "Eastern Lapita" to "Polynesian Plane Ware." This shift in terminology points to major changes in production as well as the development of new distinctive cultural elements. Eventually pottery totally disappears, though not too long after the archeological record of pottery fades away, the record of monumental stone architecture in Polynesia begins.
The abandonment of pottery does seem on the surface to be a bit strange. Indeed, Patrick Kirch writes that "ceramics are one of the most important human inventions, and to give them up willingly strikes most of us as peculiar." Kirch goes on to offer a few insights which might explain why pottery disappears from the historical record. First off, he argues that we can assume pottery was abandoned for "non technological reasons" as the islands have suitable clay and fuel-- we know this by looking at the islands today and for the reason that we have plenty of evidence of local manufacturing. So if there is not a technological reason, what is there?
Kirch sites the decline of long distance trade routes between the West/Central Pacific. With the decline in long distance trade, the prestige value of pottery, which would have been carried as trade objects, began to decline. The abandonment of intricate decoration hints at the fact that the pottery no longer fulfilled a high value or prestige role in society. As Doug Yen argues, the pottery of the central Pacific became "proletarianized." Once pottery no longer held value as a prestige or exchange good, it's value was only as a utilitarian object and so decoration was no longer important. An as a utilitarian object, pottery was easily replaced by other goods that were more readily available. Kirch cites a number of examples : earthen ovens were preferred over clay pots for cooking, when liquids like coconut cream needed to be boiled they could be stone boiled in wooden containers, wooden containers were typically more durable than clay pottery, coconut shells are water tight and more widely available than pottery to store water.
Altogether pottery seems to have been abandoned because it no longer fulfilled a cultural niche once it was no longer valuable as a prestige good. While it maintained its usefulness for cooking or storage for hundreds of years, it eventually fell out of favor to other more widely available objects. It just was not worth the time and resources to produce.
In terms of useful sources for this, I think the Kirch book I quoted above is probably the most accessible/available.