Do you have reception problems on any other TV decoders? I understand that recent weather patterns have caused interference in various areas, and perhaps depending on your aerial.
If the problem is down to recurring corruption in the tuning database, It's possible that the System Flush update will help, as it leaves you (AIUI) with clean reformatted RW flash file systems (where the relevant database lives). OTOH the flash storage itself might be on the way out, though I'm not sure anyone has actually found a case of this.
It's almost 100% not an RF issue, I am in the primary service area of Crystal Palace, and have two aerials at home, both of which feed other devices, none of which have had any issues at all.
The faulty Humax has been tried on both of them.
The signal indications on everything, including the faulty Humax when it wasn't working, were all as expected.
(I work as an RF engineer in the broadcasting industry, so am familiar with disappointing signals and mis behaving decoders!)
Thanks for the suggestions of the system flush and should the fault re-appear, I will give that a go next, and obviously report back here.
I suppose, from the specs of the components involved and the APIs used by the Humax settop binary, that tuning works like this:
the Humax settop program presents the GUI for tuning and the user selects and initiates a tuning operation;
the settop program issues a matching tuning command, or series of them, to the tuner/demodulator module (CXD2820R), perhaps using the NEXUS_I2c_* APIs;
the tuner/demodulator module executes the command(s) and sends the results back to the settop program;
the settop program offers the results to the user with a Save option;
if Save is chosen, the results are written to the /var/lib/humaxtv/channel.db[ SQLite3 database;
subsequently, to tune to a particular service (ie, "TV channel"), the settop program obtains the tuning parameters from the database (in fact, it probably loads it into internal data structures at Save or startup) and issues the necessary tuning command(s) to activate the associated multiplex and possibly also extract the requested service from that multiplex;
the selected service is then available in the MPEG-TS stream output of the tuner/demodulator module;
the settop program issues commands to the on-chip coprocessing modules of the BCM7405 settop IC to route the selected service into the Data Transport module and then to AV decoding, processing and display output, or to a recording file; this data flows using DMA between the coprocessor modules and doesn't involve the settop program itself, which is just a controller.
Although the HDR has two modules, they are effectively twins and the same tuning data would apply to both.
Based on this, corruption of the tuning database is the likely cause of missing channels that can easily be recovered, as other failure modes would be likely to break some part of the chain above.
The hidden service menu provides a way for non-CF users to save and restore configurations, including tuning and network settings. Once the system lets you reach Menu>Settings>System>System Information after it loses channel or decides to have a factory reset, you can restore your settings in that way.