Performing 31 16K prefetches with the default asynchronous prefetch limit of 10 percent of the buffers in the pool requires a pool with at least 310 16K buffers. If the pool is smaller, or if the limit is lower, some prefetch requests will be denied. To allow more asynchronous prefetch activity in the pool, you can configure a larger pool or a larger prefetch limit for the pool.
If multiple overlapping queries perform table scans using the same pool, the number of unused, prefetched pages allowed in the poll needs to be higher. The queries are probably issuing prefetch requests at slightly staggered times and are at different stages in reading the accessed pages. For example, one query may have just prefetched 31 pages, and have 31 unused pages in the pool, while an earlier query has only 2 or 3 unused pages left. To start your tuning efforts for these queries, assume one-half the number of pages for a prefetch request multiplied by the number of active queries in the pool.