How can I make my tea stronger?
You cannot make it stronger with this style of pot. You are limited by Nernst's law: the amount of solute extracted from the leaf into the tea does not depend only on the amount of tea leaves, but also on the current concentration of solute in the liquid surrounding the leaves. 

If you make tea using loose leaves in a teapot, they float everywhere, with sufficient distance between leaves. Several processes in the teapot (convection, diffusion, etc.) let the solutes move around easier, and you get an even, weak, concentration throughout the teapot, so the layer of tea surrounding each leaf has low concentration and works against the leaf from all sides, extracting lots of stuff. If you make it using a drip style method, the leaves are packed together, but the water moving through them doing the extracting is fresh and has no solute in it at all, so it is able to extract a lot. 

But with your style of teapot, the water is of course able to get into the infuser through the tiny slits, but once this has happened, there is very little exchange between the infuser and the rest of the teapot. The slits create a bottleneck, and you end up with an area of highly concentrated tea in the infuser and an area of low concentrated tea outside, with little communication between the two. The concentrated tea in the infuser can't extract the tea well. 

You could always increase the amount of tea or the time you infuse, but both methods have their drawbacks. 

Bottom line: you have to decide between well extracted tea or conveniently extracted tea. The same methods which make teapots easy to clean (concentrating the tea leaves in a small area without too much communication with the rest of the pot) reduce the extraction quality and strength.