检查UICollectionViewFlowLayout的minimumInteritemSpacingForSectionAt方法的返回值是否为0，并确保在UICollectionViewDelegateFlowLayout协议中实现该方法并返回正确的值。下面是一个示例实现：

class MyCollectionViewFlowLayout: UICollectionViewFlowLayout {
    override func prepare() {
        super.prepare()
        minimumInteritemSpacing = 0
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let attributes = super.layoutAttributesForElements(in: rect)
        var leftMargin = sectionInset.left
        var maxY: CGFloat = -1.0
        attributes?.forEach { layoutAttribute in
            if layoutAttribute.frame.origin.y >= maxY || layoutAttribute.frame.origin.x == sectionInset.left {
                leftMargin = sectionInset.left
            }
            layoutAttribute.frame.origin.x = leftMargin
            leftMargin += layoutAttribute.frame.width + minimumInteritemSpacing
            maxY = max(layoutAttribute.frame.maxY , maxY)
        }
        return attributes
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        let attribute = super.layoutAttributesForItem(at: indexPath)?.copy() as? UICollectionViewLayoutAttributes
        var leftMargin: CGFloat = sectionInset.left
        if indexPath.row > 0 {
            let previousIndexPath = IndexPath(row: indexPath.row - 1, section: indexPath.section)
            if let previousFrame = layoutAttributesForItem(at: previousIndexPath)?.frame {
                leftMargin = previousFrame.origin.x + previousFrame.width + minimumInteritemSpacing
            }
        }
        attribute?.frame.origin.x = leftMargin
        return attribute
    }

    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        return true
    }
}

extension MyViewController: UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
        return 0
    }
}