In History of Western Philosophy (1946) Russell focused mainly on two problems that relate to Kant's view that perception links our minds with "things in themselves". It is those problems, according to Russell, that caused Kant's successors to stray from his theoretical philosophy.
The first problem concerns the causal link that Kant posited between our perception and the "things in themselves". That is, Kant held that the "things in themselves" are those which cause our perceptions. However, this view starkly contradicts another basic view of Kant's: that causality is a subjective category, and as such does not pertain at all to "things in themselves". This is perhaps the most famous problem of Kant's theoretical philosophy, and all of Kant's successors responded to it.
The second problem relates to Kant's view that the real causes of perception, the "things in themselves", are not in physical space. And that it is our mind, our subjectivity, which "orders" the perceptions in physical space. The problem is that Kant did not provide a plausible explanation, on what basis we "order" the perceptions in space, by ourselves. A similar problem pertains to the ordering of perceptual events in time. The second problem influenced the more realist- minded of Kant's successors, such as Schopenhauer.
The ‘thing -in-itself’ was an awkward element in Kant's philosophy, and was abandoned by his immediate successors, who accordingly fell into something very like solipsism. Kant's inconsistencies were such as to make it inevitable that philosophers who were influenced by him should develop rapidly either in the empirical or in the absolutist direction ; it was, in fact, in the latter direction that German philosophy moved until after the death of Hegel. (p.650)