Otter, welcome to the Okanagan. The story on the big lake is that the shrimp outcompete the juvenile kokanee for food, so the kokanee populations are way below the level of when the shrimp were first introduced. Consequently, the numbers of 10lb+ trout are also down from the old days. You can still get decent numbers of 2-3 lb trout, but for fish that size, most people prefer the peace and quiet of flyfishing the many lakes on the Connector, above Summerland or Peachland, Beaver Lake chain etc...
The kokanee have rebounded slightly, and people do still catch large trout in the big lake. It is a matter of putting in lots of time trolling bucktails or spoons/plugs. Fall thru spring seems to be the best time for bigger trout.
If you are a diehard troller, Shuswap, Kalamalka and Mabel lakes may be better bets. Woods Lake is great for biggish kokanee, but they are starting to show their spawning colours already. Try Shannon Lake right in Westbank for bass, or go south to Skaha/Vaseaux.
Trout Waters Supplies in Kelowna is a great place for fly stuff, some hardware, and especially info.
If you're like me, you'll spend years exploring the various options around here, but you'll still enjoy the occasional trip to the coast for salmon/halibut the best.