Struggling on 137kHz - time to move on?

Today I did some changes to my 137kHz transverter and put it on the air this evening for about 4 hours sending and receiving 137.5kHz WSPR. Power from the PA is now around 20W and the ERP must be around 200uW.  I was really quite hopeful that, with enough stations active on WSPR tonight, I might get a few reports.  The band was indeed quite busy with a few stations like G0KTN and G3WCB who have regularly copied my 500kHz WSPR on the band and looking.  What a disappointment! Not only did I get not a single report, but I was unable to copy any of the active TX stations like F5WK or M0PPP. The latter was visible as a trace but not strong enough to decode.

Without increasing the power another 6dB at least to around 80W and improving the antenna considerably I am now of the opinion that, QRSS30 apart, 137kHz is not going to be a success. My feeling is I need at least 20-30dB better system performance (TX power and antenna efficiency that is) to even start to approach the level of reports I manage on 500kHz. So, do I try just that bit more or do I throw in the QRP towel on this band? I don't want to run lots of power and I don't want to erect a monster antenna: I was hoping my unobtrusive loop would have done me as well as it did on 500kHz, but clearly not.

I am beginning to think that this really IS a band where you need lots of power to make a success of things.

UPDATE: Just as I finished writing this I manage to decode M0PPP twice at 182km.
2252 -27 -0.5   0.137562  1 M0PPP IO93 17