Say two spaceships start at the same point and from the vantage point of your inertial reference frame $S$, Spaceship A travels at $.75c$ and Spaceship B travels at $.25c$, travelling in the same direction. The goal is to compute the velocity of spaceship A relative to spaceship B.
In the reference frame $S$, the distance $x$ between the spaceships after a particular time $t$ is given as follows.
$$x=0.75ct-0.25ct=0.5ct$$
Relative to the reference frame of spaceship $B$, there is a Lorentz factor equal to the following value.
$$\gamma = \frac{1}{\sqrt{1-0.25^2}} = \sqrt{\frac{16}{15}}$$
Thus, this same time interval $t$ is observed in reference frame $B$ as $t_B = t\gamma$ and the distance interval between the spaceships is $x_B=x/\gamma$. Thus, the velocity of spaceship $A$ in the reference frame of spaceship $B$ should be
$$v = \frac{x_B}{t_B} = \frac{x/\gamma}{t\gamma} = 0.5 \cdot \frac{16}{15} \cdot c \approx 0.46875 c.$$
However, this value is LESS than the difference in velocities of the spaceships as observed in $S$, when it should instead be more. What is incorrect about this reasoning?