Removing UniSwap Liquidity from code is not working

I have a token contract which adds liquidity using addLiquidityETH. The LP Tokens are in the contract. Now, I need a function to remove the liquidity (to shake out MEV bots).

Now, i'm wondering why this codes works (even when i pass percent = 10000, which represents 100% of the LP tokens):

function removeLiquidity(uint256 percent) external onlyOwner {
    // percent 5 = 0.05%
    uint256 lpBalance = IERC20(lpPair).balanceOf(address(this));

    uint256 lpAmount = lpBalance * percent / 10000;

    IERC20(lpPair).approve(address(dexRouter), lpAmount);

    dexRouter.removeLiquidityETH(
        address(this),
        lpAmount,
        1, // slippage is unavoidable
        1, // slippage is unavoidable
        address(this),
        block.timestamp
    );
}

but this doesn't (returns TRANSFER_FROM_FAILED):

function removeLiquidity() external onlyOwner {
    // percent 5 = 0.05%
    uint256 lpBalance = IERC20(lpPair).balanceOf(address(this));

    IERC20(lpPair).approve(address(dexRouter), lpBalance);

    dexRouter.removeLiquidityETH(
        address(this),
        lpBalance,
        1, // slippage is unavoidable
        1, // slippage is unavoidable
        address(this),
        block.timestamp
    );
}