Does anyone have a systematic or semi-systematic way of searching for these fares?

I snagged a couple of $300+ fares ex-PHL to go back in February, but had to cancel because of illness in the family. And found at least two of the ORD-IAD-LHR variety at $327 a couple of days ago, but stupidly waited to pull the trigger and it was all gone when I went back. So, I understand the basics. But I can't say that I found these fares by searching, since it was truly random. Is there no non-random way to go about it?

If you plug in your desired itinerary and dates, but the trick doesn't take, what next? It seems that for the same dates, there might be a number of flights it didn't work on, but then one that it did, with no apparent rhyme or reason why one panned out and the other didn't. So ought one go through one possibility after another, and there may be a great many, before giving up on one or both of those dates? One has no way of know whether the problem, i.e., why it isn't working, is because nothing is going to work for their given outbound date, or because nothing is going to work for their given return date, or both the outbound and return are the problem, do they? It's hard for me to accept that there is not a better way to go about it than the purely random one, but I don't see any alternatives. Is this just the way the PL website works, which is to say very, very poorly, at least for these purposes?