What is wrong with 'Det var ikke alltid så.'?
"Så" does not translate to "so" in this context.
When does it? Can you explain the different between slik and så?
In this context, you can use either "slik" or "sånn" to mean so. They're synonyms. "Så" means "so" as an intensifier.
This is odd, because I wrote "Det var ikke alltid det", and it was corrected to "Det var ikke alltid så."
Why would slikt be wrong here?
Probably because it's not considered an adjective? Et slikt problem vs. det er ikke slik. Just guessing, though...
I was thinking that since Det is neuter that one would use the neuter form slikt but apparently not...
Perhaps some examples showing the choice of usage for slik slikt and slike.