In your semilog plot, the x-axis is on a log scale, but the y-axis is linear. A linear axis doesn’t have smaller divisions if that are log numbers, so you won’t see minor ticks on it like you did in a full log-log plot. If you want both axes to follow a log scale, use loglog(x, y). Otherwise, if you want minor ticks in a semilog plot, you have to add them manually using LogLocator.