OSM highways=tracks routing issues

I have been doing my part to tidy up the OSM green lanes and bridleways in the Bristol area. I’ve come across a few routing issues where cycle.travel will not route particular highways=tracks. The nodes at each end are correctly linked but the section is always avoided. What fields does cycle.travel look at when choosing to navigate these or not? Am I right in saying it wont use routes with a mtb:scale=3 or above? 

Also, how often does cycle.travel pull through the new data from OSM.

Thanks