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.

While Rumtscho has a point regarding the ultimate effectiveness of the tea pot, looking at the picture you've posted, the teapot you're using is far from optimal with regards to the trade-off between brew strength and easy cleaning.

Your basic teapot uses a mesh to keep the leaves from floating away:

You can see how that would give a lot more exposure to the water than tiny slits!

There are even similar products that have tiny holes all over instead of slits only at the bottom:

In general, a teapot that exposes more tea to water will be able to get a stronger brew. If you try a pot like that and still can't get it strong enough, you might need to switch to drip-filter, though in general, the more tea you use, the stronger the brew will be.

The infuser you have will not give you a good result because the holes are way too small for the water to properly circulate around the tea leaves. Replace it with a wire mesh infuser, or simply put the leaves in straight and then strain them out as you pour.

A method I've liked for (somewhat) convenient, and well extracted brews is to use my French press and fresh, loose leaves. Steeping and then pressing allows for a decent extraction, and still fairly simple cleanup. After removing and rinsing the press, I add a minimal (1/4 - 1/2 cup of water) and just get the leaves swirling, then dump them in the grounds/leaves composter.

Maybe you can try to put more tea leaves and perform tea-washing job (just several spoons hot water about 60-70? to clean the impurities on leaves).

There's a few things you can do:

• Tea form: crushed tea (tea bags) will generally give you a stronger brew than full leaf tea. This is because tea bags will fully release their flavor in a single brew, while loose leaf teas release less, but last for more brews. Most likely your colleague's tea is pretty crushed, almost like coffee, which is why it releases so much flavor.
• Tea type & oxidation: more oxidized tea types such as black, oolong and pu erh, generally taste stronger than less oxidized teas such as white and green tea. This isn't relevant for you since you're already brewing black tea, but perhaps try a 'ripe pu erh'. Those usually have a really strong and thick soup, and the flavor isn't too far from a black tea.
• Temperature: brewing at higher temperatures will result in stronger teas. If you aren't brewing at boiling temperature, you can give it a try and see if it works.
• Steeping time: brew longer and you'll get a stronger brew. The extreme end of this is cold brewing, where tea brewed with cold water, but applying a super long steeping time to compensate for this, see for example this video: https://www.youtube.com/watch?v=s0Oyg_xWjZ4
• Teaware material: ceramic, porcelain and clay teapots are worth to try out, because they isolate heat better than glass teapots. This also helps you to draw more flavor out of teas.