Date: 201606
Local time of day: 1201-1800
Location: ZOA.ARTCC, CA
Flight conditions: VMC
Light: Daylight
Aircraft: Sail Plane
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Personal
Flight phase: Cruise
Airspace: Class A ZOA; Class E ZOA
ATC / advisory: Center ZOA
Reporter organization: Personal
Reporter function: Instructor; Pilot Not Flying
Reporter qualification: Flight Crew Glider; Flight Crew Flight Instructor; Flight Crew Commercial
Second reporter function: Pilot Flying
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Diverted
Contributing factors: Weather; Human Factors; Airspace Structure; ATC Equipment / Nav Facility / Buildings
Primary problem: Ambiguous

Narrative: We were flying mountain wave conditions [along the eastern Sierra Nevada]. We had prearranged a clearance for operations inside R-2508 in Class A airspace. The weather conditions had been hugely favorable [for several hours]. We had to work to remain below class A airspace. We had worked successfully on two clearances with Oakland already on this day. The first Oakland clearance was approximately an hour in the morning and the second clearance was 90 minutes in the afternoon.On our next leg southbound; we asked OAK for another clearance. They were very cooperative; and gave us a block altitude assignment; FL170 to FL230. The frequency was busy with traffic at this time. We encountered a loss of lift; and could not reestablish a positive climb rate. We asked for a lower clearance and were told MEA was 14;500 MSL for terrain separation; 'Could we ensure terrain separation?' Yes; we replied. We heard another aircraft diverted around us for separation. We tried to ask and confirm our lower block assignment and couldn't contact OAK. We tried to cancel IFR and couldn't contact OAK. OAK continued to try to confirm us to cancel and another aircraft confirmed to OAK that we were hearing their calls; answering and trying to cancel our clearance. We had to turn northbound to retain glide to a runway; given the rate of sinking air mass. The relay pilot confirmed to OAK that we wished to cancel IFR. The problem occurred partially because we anticipated a similar air mass to our two prior passes through the area on this day. Secondly; we didn't anticipate a loss of radio contact at 14;500 MSL; nor a great density of IFR traffic in the region to create a 'near miss' situation. The congestion on the frequency contributed to the delay in our request to cancel IFR. We have no idea how close the [other aircraft] was to us; we never saw that aircraft.How do we prevent this in the future?For ATC; for gliders to not authorize a lower floor of block assignment; to come near to the floor of radio coverage in the sector. For me as a pilot; to not request a block clearance unless we are established in extremely strong lifting conditions; and can maintain more than 1000 ft above our floor of the block. We should cancel immediately; if the trend is penetrating the interior buffer on our floor.


Second narrative: [Report narrative contained no additional information.]


Synopsis: Sailplane instructor and student reported lifting conditions diminished greatly and the pilots were unable to maintain their block assignment or contact ATC directly. The pilots turned back toward the departure airport before ATC clearance could be received.