Which Drobo do you have? model?
How is it made available to PMS? usb / network share?
Does the problem persist if you reboot your Mac and Drobo?
Can you try to stream the film with your mac as a file without using Plex?
What transfer rate can you get from the Drobo?
When I played hero 6 my PMS log included the following:
May 14, 2017 18:12:35.382 [0x7fe0607ff700] DEBUG - Content-Length of /media/raid/Plex/Films/big.hero.6.mkv is 5867951866.
May 14, 2017 18:12:35.809 [0x7fe081fff700] DEBUG - Auth: We found auth token (xxxxxxxxxxxxxxxxxxxx), enabling token-based authentication.
May 14, 2017 18:12:35.809 [0x7fe081fff700] DEBUG - Auth: authenticated user 1 as scorpuk
May 14, 2017 18:12:35.809 [0x7fe081fff700] DEBUG - Auth: Came in with a super-token, authorization succeeded.
May 14, 2017 18:12:35.809 [0x7fe0763ff700] DEBUG - Request: [192.168.1.1:58758 (Subnet)] GET /:/timeline?hasMDE=1&ratingKey=13102&key=%2Flibrary%2Fmetadata%2F13102&state=playing&playQueueItemID=24740&time=208&duration=6112649 (16 live) TLS GZIP Signed-in Token (scorpuk)
May 14, 2017 18:12:35.810 [0x7fe0763ff700] DEBUG - Client [eqsjtfg0kmx3ach3150bxzux] reporting timeline state playing, progress of 208/6112649ms for guid=, ratingKey=13102 url=, key=/library/metadata/13102, containerKey=, metadataId=13102
May 14, 2017 18:12:35.825 [0x7fe05dfff700] DEBUG - Created thumbnail of size 1280x720, has pixels: 1
May 14, 2017 18:12:35.839 [0x7fe0763ff700] DEBUG - We’re going to try to auto-select an audio stream for account 1.
May 14, 2017 18:12:35.839 [0x7fe0763ff700] DEBUG - Selecting best audio stream for part ID 13451 (autoselect: 1 language: en)
May 14, 2017 18:12:35.839 [0x7fe0763ff700] DEBUG - We’re going to try to auto-select a subtitle.
May 14, 2017 18:12:35.839 [0x7fe0763ff700] DEBUG - Audio Stream: 21958, Subtitle Stream: -1
May 14, 2017 18:12:35.840 [0x7fe0763ff700] DEBUG - [Now] User is scorpuk (ID: 1)
May 14, 2017 18:12:35.840 [0x7fe0763ff700] DEBUG - [Now] Device is Konvergo (DESKTOP-JGULB56).
May 14, 2017 18:12:35.841 [0x7fe0763ff700] DEBUG - [Now] Profile is Konvergo
May 14, 2017 18:12:35.841 [0x7fe0763ff700] DEBUG - [Now] Updated play state for /library/metadata/13102.
May 14, 2017 18:12:35.841 [0x7fe0763ff700] DEBUG - Statistics: (453jrfkbq4y63vng7x0q7zxg) Reporting active playback in state 0 of type 1 (scrobble: 0) for account 1
May 14, 2017 18:12:35.846 [0x7fe0763ff700] DEBUG - It took 0.0 sec to serialize a list with 0 elements.
May 14, 2017 18:12:35.847 [0x7fe0817fe700] DEBUG - Completed: [192.168.1.1:58758] 200 GET /:/timeline?hasMDE=1&ratingKey=13102&key=%2Flibrary%2Fmetadata%2F13102&state=playing&playQueueItemID=24740&time=208&duration=6112649 (16 live) TLS GZIP 37ms 421 bytes (pipelined: 10)
May 14, 2017 18:12:35.871 [0x7fe0817fe700] DEBUG - Auth: We found auth token (xxxxxxxxxxxxxxxxxxxx), enabling token-based authentication.
May 14, 2017 18:12:35.871 [0x7fe0817fe700] DEBUG - Auth: authenticated user 1 as scorpuk
May 14, 2017 18:12:35.871 [0x7fe0817fe700] DEBUG - Auth: Came in with a super-token, authorization succeeded.
May 14, 2017 18:12:35.871 [0x7fe061bff700] DEBUG - Request: [192.168.1.1:58759 (Subnet)] GET /status/sessions (16 live) TLS GZIP Signed-in Token (scorpuk)
May 14, 2017 18:12:35.871 [0x7fe061bff700] DEBUG - [Now] Adding 1 sessions.
May 14, 2017 18:12:35.872 [0x7fe061bff700] DEBUG - It took 0.0 sec to serialize a list with 1 elements.
May 14, 2017 18:12:35.873 [0x7fe0817fe700] DEBUG - Completed: [192.168.1.1:58759] 200 GET /status/sessions (16 live) TLS GZIP 1ms 2180 bytes (pipelined: 9)
May 14, 2017 18:12:35.963 [0x7fe0817fe700] DEBUG - Completed: [192.168.1.1:58754] 200 GET /photo/:/transcode?url=%2Flibrary%2Fmetadata%2F13102%2Fart%2F1493260022%3FX-Plex-Token%3Dxxxxxxxxxxxxxxxxxxxx&width=1280&height=720 (16 live) TLS GZIP 795ms 246302 bytes (pipelined: 12)
May 14, 2017 18:12:39.620 [0x7fe053fff700] DEBUG - Streaming Resource: Updating client bandwidth to 33323kbps with most recent 33323kbps (4177920)
May 14, 2017 18:12:40.035 [0x7fe05f3f7700] DEBUG - Streaming Resource: Updating client bandwidth to 128235kbps with most recent 1228357kbps (360448)
May 14, 2017 18:12:40.383 [0x7fe05f3f7700] DEBUG - Streaming Resource: Updating client bandwidth to 212435kbps with most recent 1272582kbps (360448)
May 14, 2017 18:12:40.808 [0x7fe053fff700] DEBUG - Streaming Resource: Updating client bandwidth to 282775kbps with most recent 1334359kbps (327680)
May 14, 2017 18:12:41.226 [0x7fe052fff700] DEBUG - Streaming Resource: Updating client bandwidth to 343523kbps with most recent 1224377kbps (360448)
Following your log from similar point at 17:59:23.565, I dont see the bit from mine in italics…(…DEBUG - Completed: [192.168.1.1:58754] 200 GET /photo/:/transcode?url=%2Flibra…)