There are some positions, in which both players have pawns, rooks, and even queens, but checkmate cannot be forced.
[FEN "4k3/4r1r1/3p1p2/p1pPpPp1/PpP1P1Pp/1P5P/8/4K3 w - - 0 1"]
For instance in this position both players have pawns, and black has two rooks, however in order for either player to get checkmated black first has to sacrifice a rook and white has to capture it, however both sacrificing the rooks, and capturing the rooks can be avoided.
[FEN "4b1NQ/3bPpPP/5P1P/2K5/4k3/8/8/8 w - - 0 1"]
And in this position white has a queen, pawns, and a knight, but so long as black doesn't move the bishop on e8, and white doesn't move the pawn on e7, neither player can ever get checkmated, and black can avoid moving the bishop, and white can avoid moving the pawn, so neither player can force checkmate.
[FEN "4K3/4Q3/8/8/8/8/4q3/4k3 w - - 0 1"]
And in this position each player has a queen, but we can see that black cannot have a forced mate as white can just take the black queen, and also white cannot have a forced win as if white takes the black queen the black can take back, and if white moves the king then black can capture the white queen.
So using USCF rules would the game be declared over if it reached any of these 3 positions?