"The aircraft must be 60% or more below the GS centerline during the approach to the intercept point"
I've been similarly baffled by apparently intermittent success in coupling an STEC 60-2 to an ILS GS, which has a similar requirement. I think the condition you quote is the key. Thank you madlandrover, you've saved me some head scratching.
cjboy, as you've probably gathered from the manual, the STEC series have no "approach mode" as such, but the NAV mode flies the glideslope (and illuminates the APR light).