Tvheadend

After successfully recording a number of programmes yesterday, it missed two on BBC 4 last night. This happens frequently so I'd appreciate some help in understanding the log file. My TV adaptor is a Hauppauge WinTV Nova-TD which I've configured to appear as DiBcom 7000PC A and DiBcom 7000PC B. I don't know if it's significant that the second programme appeared to fail in a different way from the first: was it using adaptor B before the first failed recording had released it?

What is the meaning of "No input detected" and what can I do about it?

Log file fragment from end of last successful recording:

Dec 03 20:30:00.012 [ INFO]:dvr: Recording completed: "/tv/Coronation Street.ts"
Dec 03 20:30:00.013 [ INFO]:dvr: "Coronation Street" on "ITV": End of program: Completed OK
Dec 03 23:24:30.018 [ INFO]:dvr: "When Albums Ruled the World" on "BBC FOUR" recorder starting
Dec 03 23:24:30.022 [ INFO]:subscription: "DVR: When Albums Ruled the World" subscribing on "BBC FOUR", weight: 300, adapter: "DiBcom 7000PC B", network: "N
orth West", mux: "North West: 706,000 kHz", provider: "", service: "BBC FOUR", quality: 100
Dec 03 23:24:42.019 [ ERROR]:dvr: Recording unable to start: "When Albums Ruled the World": No input detected
Dec 04 00:00:32.094 [ INFO]:dvr: "The Fall" on "BBC TWO" starting at 2014-12-11 21:00:00, scheduled for recording by "Auto recording by: adminpete"
Dec 04 00:01:01.318 [ INFO]:dvr: "Newsround" on "CBBC" starting at 2014-12-11 16:20:00, scheduled for recording by "Auto recording by: adminpete"
Dec 04 00:54:30.019 [ INFO]:dvr: "Blondie: One Way or Another" on "BBC FOUR" recorder starting
Dec 04 00:54:30.019 [ INFO]:subscription: "DVR: Blondie: One Way or Another" subscribing on "BBC FOUR", weight: 300, adapter: "DiBcom 7000PC B", network: "North West", mux: "North West: 706,000 kHz", provider: "", service: "BBC FOUR", quality: 100
Dec 04 00:55:00.020 [ INFO]:subscription: "DVR: When Albums Ruled the World" unsubscribing from "BBC FOUR"
Dec 04 00:55:00.022 [ INFO]:dvr: "When Albums Ruled the World" on "BBC FOUR": End of program: Time missed
Dec 04 02:05:00.008 [ INFO]:subscription: "DVR: Blondie: One Way or Another" unsubscribing from "BBC FOUR"
Dec 04 02:05:00.009 [ INFO]:dvr: "Blondie: One Way or Another" on "BBC FOUR": End of program: Time missed

Environment:
HTS Tvheadend 3.4.28~geb79aee~trusty
Gigabyte Brix GB-BXBT-2807 Celeron N2807 1.58/2.16GHz dual-core 4GB RAM 128GB SSD
Linux Mint 17 "Quiana"
The machine is dedicated to running tvheadend: no other user code running concurrently.

Regards, Pete