Trouble finding slime chunks

I'm looking for slime chunks and not finding any. I think I'm doing everything right, but out of 20-30 chunks checked, I haven't found one yet. Here's what I'm doing.

-- I'm mining at level 6, finding chunk borders with F3, digging out 5x5 rooms 4 high in each chunk.
-- I'm lighting with glowstone recessed in the ceiling.
-- I'm in an area with little or no caves or mineshafts.
-- I'm lighting my tunnels with torches.
-- I'm mining in an area that takes me outside a 24-block radius of each room as I work. I'm also outside of 32 blocks frequently as I work, so there may be spawning happening too.
-- I check rooms frequently, every 5-20 minutes depending on what I'm doing.
-- I've read all the forum posts on finding slime chunks.

Any ideas or suggestions are very appreciated. If you've found slime chunks before, how did you do it? Does anyone think they know why with a 1 in 10 chance of finding one and close to 30 chunks searched I still am not finding one?

I made a massive mine once, and found a strategy that helped me spawn tons of slimes. Here's how you do it:

1. Grab a bunch of E5 picks.
2. Set up a haste 2 beacon near wherever you want yer farm to be.
3. Go crazy. Mine anywhere and everywhere, digging out 3 y levels.
4. Light up the way as you go.
5. If slimes start spawning, make a boarder around each chunk so that slimes can't escape.
6. Find the chunk that spawns slimes, and farm it!

Here is what I normally do since we can't use mods to find chunks.

Dig out a huge area with a 3 block gap from ground to roof. When you see a slime spawn, use fence to mark out that chunk. Then if you see more spawn inside that fenced area, you know for sure it's a slime chunk. Do this over and over until you have many confirmed chunks, then use the areas that have the most chunks together as a farm. Just make sure to be at least 25 blocks away from closest slime chunk. I always use pumpkins to light the ground, not sure if it's any difference between that and glowstone. Just never torches.

Thank you so much for your responses. While I was typing this, one slime spawned in one of my chunks. Yay!

I think perhaps I wasn't being patient enough. But good to know my method worked too. Hoping more slimes will spawn in other of my chunks as well.