Bulk API using R elastic package

I'm indexing a series of bulk files, around 10 MB each. Most of them upload fine, but sometimes I don't get a response from Elasticsearch. It'll just sit there as if it's still processing. I can see from Sense that around 90% of the documents from the offending files are indexed correctly. I'm working in R using the package 'elastic' I've also tried doing a POST to /_bulk with the upload file, and it does the same thing. Does anyone know what could be causing this?

edit: after about an hour I got a response which flashed some 400 errors and then my R session crashed.

This is the error I see when I send the request in Fiddler. I basically have no idea what it means.
error: "ElasticsearchParseException[yaml does not support stream parsing...]"
status: 400