This is strange for me - and now I need to ask my Japanese acquaintances the same questions!
I'm able to visualize the Chinese characters I know just fine, and I'm by no means fluent. I speak Japanese instead of Chinese, but I don't think that would make a difference when visualizing the characters since most of them are one in the same.
I can visualize「雑」(from: 複雑) because of my familiarity. I struggle with 「爆」, a character I just recently learned, because I am not familiar with it. I would have thought that natives of Chinese or Japanese would have no difficulties with visualizing the characters! So it is interesting for me to hear anecdata that they struggle with it.
E:
Reading Xophmeister's reply it actually makes sense to me. For myself, Chinese characters are much more artistic and it is like visualizing a familiar painting or drawing. For a native, they are closer to a tool used for writing, like how English is for myself. For English, I can only visualize words with eight or nine characters or so. It's a weird practice to try and visualize the word "adenohypophysis" - it is difficult. So I can imagine any non-trivial Chinese character would be difficult to visualize for native speakers.
E2:
Replaced "Kanji" with "Chinese characters" in a few places it slipped through.
To me "kanji" is an English word but "hanzi" isn't. I never saw any problem using "kanji" in a Chinese context, since "kanji" seems to be a Japanese sounding of the word "hanzi". But unfortunately some pedants (or one pedant with many usernames) complained when I used "kanji" so nowadays I stick to "Unihan" when writing in a technical context.
AFAIK the historical reasons are well-placed. Japanese Kanji is essentially a spin-off system, since altered, wherein many of the characters that exist in both Chinese (eg. in the Tang Dynasty) and Japanese now have completely different modern meanings. In addition, grammars in both places have since changed (Japanese possibly never really had one, deferring to Middle Chinese in early use, and only slowly adopting characters elsewhere, interspersing other writing systems to provide grammatical structure). Therefore it makes little sense to conflate the two when discussing language, though the evolution of Japanese character use can provide some insight in to Middle Chinese and earlier periods, particularly in subjects such as Nara Period and Silk Road Buddhism. (I am less knowledgeable about Japanese than Chinese, and claim no particular expertise in either, though I do translate old Chinese texts to English for fun and have visited Japan)
I'm able to visualize the Chinese characters I know just fine, and I'm by no means fluent. I speak Japanese instead of Chinese, but I don't think that would make a difference when visualizing the characters since most of them are one in the same.
I can visualize「雑」(from: 複雑) because of my familiarity. I struggle with 「爆」, a character I just recently learned, because I am not familiar with it. I would have thought that natives of Chinese or Japanese would have no difficulties with visualizing the characters! So it is interesting for me to hear anecdata that they struggle with it.
E:
Reading Xophmeister's reply it actually makes sense to me. For myself, Chinese characters are much more artistic and it is like visualizing a familiar painting or drawing. For a native, they are closer to a tool used for writing, like how English is for myself. For English, I can only visualize words with eight or nine characters or so. It's a weird practice to try and visualize the word "adenohypophysis" - it is difficult. So I can imagine any non-trivial Chinese character would be difficult to visualize for native speakers.
E2:
Replaced "Kanji" with "Chinese characters" in a few places it slipped through.