one of the major inhibitors are, indeed, the "nuclear secrets" as it were. there are a few special sauces that just take experimenting and tinkering and brainpower to work out. all of which also cost more and require more supplies.
or, at least, thats my simplified, non-physicist understanding of it.
I remember reading about a US research project where someone with reasonable physics knowledge, but no specific knowledge on the workings of atomic bombs, managed to put together viable plans for a bomb from only publically available information.
The second is "fast enough", since if you don't assemble the material fast enough and hold it together for long enough it will blow itself apart before there is a chance for the chain reaction to build, and you will have a dud (at least compared to the intended yield). To get around this problem you will probably need some fairly advanced shaped charges and a properly designed tamper. This is what the "nuclear secrets" cover. Obviously it is possible to derive this information for oneself, but it's non-trivial to say the least.
how do you ensure that in that collision, how to ensure that the "slamming" consumes enough of the uranium, converting it into boom, before the boom destroys the weapon itself, halting the reaction.
but again, not an expert. i'd love to hear more on it if you or someone else are.