Plotting isocline for a non_linear system o equation

I have a non-linear system of equation

dx/dt =-x + phi(w*x - y -0.5)

dy/dt = -y + phi(x - y -0.5)

phi = x^2 / ((0.75^2) + (x^2))

I know for example for w=4 I have 3 roots(real), but for y, I get a straight line, so always one point of interaction and one root. How can I correctly plot Isocline of that?

Thank you

[Bhavana Ravirala]

Could you share the initial conditions of the equation, that you used in your code.