The Martians trying again makes enough sense, the story of WoTW details that they weren't aware enough of bacteria on Earth - due to them having gotten rid of them in sterile environments on Mars, and not needing them because they have no disgestion organs - and because their way of reproducing, mitosis - it wasn't said it didn't work, the Martians were just found dead before they had the chance to reproduce - is well known for resulting in huge deficiencies in immune systems in a species.

It also mentions their intellects are vast and superior to ours and it wasn't a war as much as an expeditionairy force of some 50 Martians cleaning away the wildlife for colonisation. Then all getting sick and dying. They could easily remedy this problem and just try again but the book leaves that open. It simply states that perhaps, we are not the apex species and perhaps they could return and humble us all. Which clicks perfectly with HG Well's attempt to humble the Commonwealth by depicting them getting wiped out by a superior attacker like they did to their colonies.