To increase the income from your castle you need to speak with the mayor and appoint a merchant, priest, watchman, etc. Each of these will give an additional income of around 400$. Also, as long as your castle is not under siege, the prosperity will increase over time. A castle at his highest status gives around 3990$, which is lower than a town (which gives around 7000-15000) but its still enough to pay for garrison and get a profit.
As for obtaining another fief, the chance of the king assigning one to you is around 20% + 1% for every 5 points of renown + 0.5% for every 25 reputation with the king - 10% for each additional fief you own. The best you can do is keep capturing cities on your own (you're not eligible for receiving a fief if another lord captured it or if you just joined a siege started by someone else) )and defeating strong lords, you will eventually get another one.
Also, if you cant leave poland try advancing the storyline a little. Kassimir eventually gives you the choice to leave service and join the khans later on.