Are you using a button field to create the url for the form?
If so, what is the field type of the field that you are using to get the linked records? You cannot prefill multiple linked records using the original linked record field. You must use a rollup with the following formula:
By the way, I wrote the Prefilled Forms app to generate the prefilled url. You can find more info about using this app and how it applies to prefilling linked records on my website.