I have used several Yaesu radios over several years on these bands. None of them, possibly due to the way frequencies are synthesised in the radios themselves, have been accurate enough at 6m or 4m to place a Wspr signal accurately in the Wspr window. The higher the frequency the greater the error. I believe this to be common in many radios, not just Yaesu. The issue can be compounded by transverters.
My old faithful Yaesu FT847 for instance must have its dial frequency set 150hz low to put it within a couple of hz of where it needs to be on 4m. Given a window of 200 hz only this can mean transmitting outside of the window if the radio were centred and no adjustment made. Happily I have been aware of this from the start and as the radio is very stable can be certain it is on frequency. It does mean frequency hopping is out though unless I stay on HF were the problem is far less noticable.
I have noticed many stations on 6m who appear to be outside of the window. I have verified my radio with two separate station who have access to GPS disciplined receivers.
Perhaps many do not realise inaccuracy of transmission by a hundred hertz or more may be the reason you are not being spotted. Obviously make sure your clock is accurate first . If you still aren't spotted perhaps, as I was once, you are out of the window! Go to 0% tx and observe for a while then adjust the radio until your window agrees with average spots of others more or less.
Useful tip: given to me by Jim, look at your average report in the 'activity' menu and compare it with your set frequency in the Wspr window. This can suggest whether you are high or low. Obviously this relies on a transmission spotted within the window, and also on the accuracy of those spotting. It is a useful guide. It is worth spending some time doing this on several bands to build up a picture of your radio's accuracy. The FT847 is only a few hz out on 160m for instance.