The last two trips I tried to use SWUs on, the outbound (LGA(Y)-DFW-PEK, JFK-LAX-HKG) was available for SWU at booking, but not return, which was true throughout Dec 2017/Jan 2018. (Is this a general pattern? If so, why?)

While my return PEK-ORD cleared a day or two before departure, my HKG-LAX was a nail-biter, and did not clear until 1 hour prior to departure. I was already mentally prepared to take my Y seat. Monitoring availability on EF in the week leading up to HKG-LAX, J buckets fluctuated up and down, mostly down. Seat map showed 6 seats, then 8 seats, then back down to 6 seats unassigned. MCE did not show many seats become available (indicating possible upgraders, 2-3 aisle/window seats became available in the week prior). C never became available while monitoring or via alert. F also fluctuated between 1-3 available for upgrade or miles. Day of departure, it was J2 R1 D0 I0, or something similar.