Witbier - stuck fermentation

[coder]

My witbier:

Belgian Pils 1 kg
Wheat (raw) - 2.50 kg
Munich - 0.25 kg
Oats - 0.13 kg
Wyeast Witbiet starter 2L

It didnt ferment too much on primary, just ~50% attenuation (11.2 Plato->5.4 Plato). I knew there can be problems with such beer, I've made quite a large starer, kept the fermentor warm (21*C), I agitated fermentation twice during primary, but after 9 days I had only 5.4 Plato. I decided to rack it on secondary, hoping it will gain some 1-2 Plato on secondary, but it seems that the fermentation is over. What can I do now? Leave it underfermented, add new starter made of slurry, or even some more aggresive yeast (Czech Pils?) ?
And - what can I do to avoid such problems in the future, I'm planning a series of Wits.

[coder]

On secondary the Witbier fermented one more degree, not it has 4.5 Plato, not so bad. I'm a bit suprised, because I didn't notice any activity in airlock during secondary...

[slothrob]

Witbear yeast are infamously slow.