stuck fermentation, or unfermentable sugars?

[NorJerseyHomebro]

Made a rhubarb belgian ale recently, OG was 1.056, checked the gravity a week after fermentation started and it was only 1.020, checked again two weeks on and it is still 1.020. the sample tastes pretty balanced and dry from the rhubarb (theres about 3lbs in it). So my question is, is the fermentation stuck because that is pretty terrible attenuation or is my FG 1.020 because of unfermentable sugars from the rhubarb?
thanks in advance homebros.
-Matt

 

[magnj]

It's probably just done.