Block.One recently merged some new REX updates into the eosio repository on Github. These updates are consistent with the recent issues we have witnessed in the mainnet, where it was no longer possible to borrow resources since there was no liquidity left in the REX pool.
*"The proceeds from CPU and network renting, RAM trading fees, and name bids are currently added immediately to the REX pool, thus instantly increasing the value of REX shares. We change this pattern and channel the proceeds linearly with time over a period of 30 days. In order to achieve this in a CPU- and RAM-efficient way, we accumulate the proceeds accruing within 12-hour intervals into buckets. At the end of each interval, we start to channel the proceeds in the bucket to the REX pool linearly over 30 days. At the end of the 30 day period, the bucket is deleted. At any point in time, there can be up to a maximum of 60 buckets.
Another change is the total_unlent lower bound. It is changed from 20% to 10% of total_lent."*
From EOS Nation