In announcing this victory to Congress, Washington, though he cited almost every other major-general who was with him in battle, did not mention Lafayette’s name. This was the first time Washington had failed to do so in recounting any action in which Lafayette had taken part. Hamilton and Laurens also listed those who had distinguished themselves, and Lafayette was not among them. Dr. James McHenry, another of Washington’s secretaries, likewise called the roll of Monmouth’s heroes. Of Lafayette he said only that the marquis was ‘sadly disappointed’ and 'peculiarly unhappy’ not to have won any laurels that day (for 'the honors of war….have a distinguished place in the breast of a French noble-man’); but he added that, after the retreat was checked, Lafayette’s conduct of the second line was 'very judicious.’ A few months later Washington was to feel called upon to write a letter recommending Lafayette to Benjamin Franklin. In doing so, the commander-in-chief listed the several actions in which Lafayette had fought under his command and acquitted himself gloriously. The battles of Brandywine, Gloucester, and Barren Hill were cited; but Monmouth Court House was passed over in silence.
— Lafayette In America by Louis Gottschalk. This has always been odd to me. Was it that Lafayette drove his troops too hard on their march to the battlefield in the preceding days? Was it that Lafayette was one of the first officers to quit the field (after arguing with Lee that they should not do so) because he was ordered to fall back? Was it something he did during the battle? Was he afraid or did he not want France to know of a battle he didn’t perform spectacularly well in? Hm.