Hmm... I've never had a problem with it setting up. Here's the process I usually use, in
excruciating detail:
- Grab a container for boiling---usually a mason jar or heavy glass.
- Scoop out the amount of mix I want to prepare and dump it in the container.
- Measure and add the water (4x mix volume). I usually use warm water, but I'm not sure if that matters.
- Stir vigorously and check for clumps. Let it sit for 30 seconds or so while I set out my slates or lids. <<< This may be the part that I didn't explain very well in the instructions.
- Place in microwave and heat on full power.
- WATCH THE MIX every second while it is heating, because it can boil over quickly
- Let it boil for 10 seconds or so---less for tiny amounts, more for larger amounts. If it bubbles up too quickly, this may have to happen in short burst. Keep an eye on it and pause the microwave when necessary.
- Pull out the jar (with pot holders!) and quickly pour it onto the waiting slates and lids.
- Rinse out the jar.
- Go do something else with the intention of coming back in a few minutes.
- Forget what I was doing.
- Remember the gel sometime later (usually 10 or 15 minutes). Go poke it. It's usually set by now and feels "rubbery," but if it's soft I may stick it in the freezer for a few minutes.
- Pace around the kitchen. Open the freezer several times to see if it's cool yet.
- When it's cool, dump it in the tank and watch the fishies.

EDIT: It may also be helpful to shake up the container of mix before you scoop any out, in case the ingredients have settled and you aren't getting enough agar or something.