Flight Crew Air Transport Pilot (ATP)
Communication Breakdown; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Undershoot; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Became Reoriented
Aircraft; Human Factors
Center gave us a descent from FL380 down to FL340. First Officer (FO) was Pilot Flying. We selected 340 on MCP; we both verified; he selected ALT Intervention; the aircraft began to pitch over; and we both verified 'VNAV PATH'. I am 99.9% sure that we both saw and verbalized we were in VNAV Path and starting down. A few minutes later ATC ask us if we were descending? I looked up to see that we were still at; or close to FL380; MCP was in V/S mode. We had not descended. We corrected flight path and descended to FL340.We (I) surely missed the monitor portion of VVMI. Simply watching closely what the aircraft was doing AT ALL TIMES would have solved the problem. We continued that leg; and flew one more to ZZZ. We noticed that when given an altitude change; sometimes the MCP would stay in VNAV; sometimes it went to ALT Hold; and once to V/S. We learned that we had no idea what the aircraft was going to do; or why it was going to do it. I needed to be more involved in the monitoring of the aircraft itself. However; I found myself very; very task saturated with new hire FO; who spoke and understood English well; but somehow had a disconnect understanding or reading back ATC instructions. I think he was listening in English; thinking and translating in [a foreign language]; then translating back to English. Too much going on in his head; very poor results. I found myself very busy clearing up even simple instructions and communication problems; along with the normal; or sub-normal; new hire task loading. I was quite simply doing the best I could; but came up short.
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A B737 flight crew reported that ATC gave them descent clearance and the aircraft failed to descend using automation.