他们走远了，咱们跟不上了。

We won't catch them up, they're already way ahead.

他跟不上班。

He is struggling in class.

跟不上时代

fail to keep up with the times

跟不上形势

fail to keep a hold on the situation

跟不上形势发展

fail to keep up with the development of a situation

他的体质还跟不上我呢。

His physique isn't even as good as mine.

他可跟不上专家

he can't compare with a specialist