SH, just a last quick message for you here...the combined "Oxford Dictionaries" website you provided a link to does, indeed, refer to 'cannot seem'; however, The Oxford English Dictionary ITSELF makes it clear that it is simply a colloquial usage. (OED quote: "Also (colloq.), in negative contexts (preceded by can): to seem unable.") So, it is no more "proper" English than any other such usage. And yes, Merriam Webster could be expected to list it, given that it is an American idiom.
I have no intention whatsoever of entering a discussion of what "proper English" IS!
Most contributors here, including Count-A-Strong, Calibax, JNO, Scotman, Mike1111, Whiskeryron, MarkRoe Ludwig (initially!) as well as me are, I imagine, Brits and we seem virtually all to feel that it is not proper BRITISH English.
In conclusion, I'd be perfectly happy if you SAID (to an American), "I cannot seem to understand..." but not so ecstatic if you WROTE it (to a Brit.)
Cheers, ma'am.