was seeing its the last one in the category, and decided to do it and need at least 10k more fish. but been fishing for almost an hour and doesn’t seem to be applying the credit for it. any ideas?
You may want to try the support forums.
This is a realm forums.
Are you fishing in pools? it only counts from pools.
Also just in case you aren’t aware, because my guild is also working on this, there’s a pandaren NPC near the river next to halfhill that will tell you the best fishing spot in pandaria for the day and there will be nonstop pools in the location he tells you. 
I am not certain this is relevant for this particular achievement, but some guild achievements require the character to be exalted with the guild before their progress counts. This is to prevent people from just inviting characters with various notable things done to give the guild credit, then leaving the guild.