You lack nobility and are seen in the company of too many dirty peasants and greedy burghers.

Yes what wisnie788 is true along with building to help increase the nobility population

Thanks for the help. What buildings help increase Nobility and where can I see what my nobility level currently is?

There's a cap on those units because they're elite. You can only recruit but one or two in a single castle.

If you want more in your army, recruit them from different castles.

You must build a Citadel (Level IV Castle) to recruit them and yes they are capped so you don't go unrealistically hiring half your army as knights. You want LOTS of knights? Build the church for military units and then you can get Templar and Hospitaller knights with even better stats!! Oh...and don't forget the Ritterbruders (spelling?)

If you want to see your amount of nobels, peasants etc. there is a icon in the left bottom of the window if you click on a city. (In that line where you can see your public order buffs/debuffs etc.) I just discovered it yesterday. You can click on it to scroll to each population modifier.
Sidenote: under circumstances there is a button in the same direction allowing you to create a vassal of an already wiped out faction in a conquered city under some circumstances. Great thing if you play as the Holy Roman Empire