this is a very common complaint of all net users.
Basically, tatkal counters all over India open at 08am. so there is a lot of pressure at the servers. counters, batch files of agents, plus irctc users all start using at 0800 am. Apart from all these people, counters, and other non-tatkal bookers also use the servers.
irctc users have an additional problem of bandwidth, bank site diversion for payment of card/banking and then back to irctc servers. the normal users also use at that time.
I...
more... would suggest you to either book tatkal tickets from the resvn counter or thru Authorised E Ticket IRCTC agent. there are more chances in the alternative options than irctc website at 08am. you can still book a ticket after 0820-0830 but by that time, your tatkal tkt would be in WL.