The core developers are investigating several community reports (Discord, Github, Reddit, etc.) of Hotspots failing to stay in sync and thus not participating in PoC challenges. We've quantified this as currently affecting 13% of the network so we're working diligently on it on several fronts.
Two major lines of work are being looked at:
1. Additional improvements to both the miner and seed node side to improve network connectivity, especially for LTE/4G connected Hotspots. We made some ARP improvements a few weeks ago which helped but with the growth of the network, we need to continue to work on this.
2. Continue to improve snapshot loading for all Hotspots. Snapshots are a way for Hotspots to catch up to a more recent block in the chain if it falls out of sync but as the network expands, and more transactions are in block, this gets slower on some Hotspot hardware. There are improvements we are working on here that should continue to help them along. One thing to note is that some Hotspots (including original Helium hotspots) will clear their local storage and reload from a Snapshot when their disk becomes full. This is expected but causes them to "randomly" fall out of sync. Better Snapshot loading (and more recent Snapshots) will improve this.
Also, any Hotspot failing to stay synced can cause them not to be able to witness challenges and prevent them from sending challenges as beacons. If you are seeing dramatic changes in witness counts, this could be the reason in your area (in addition to normal fluctuations due to environment or networking).
The best way to verify your Hotspot is affected is to directly connect to the Hotspot locally over Bluetooth and check its block height directly (and compare against the block height on Explorer). If you're over 360 blocks from the chain, you're out of sync. Since these issues also affect gossip (the way Hotspots and other network nodes talk to each other), apps like the Explorer and third-party apps may be delayed in having the most up to date information. At this point, there's no action required on the part of Hotspots owners except for patience as we work through the above two issues.