Missing isochronous packets

I'm trying to transmit 64 byte isochronous USB packets over a USB high
speed link from a SAM3X (in an Arduino Due) to a libusbk program (basically
the example benchmark program it comes with).

I have one isochronous endpoint on the device and it sends data in a tight
loop like this (using Atmel's SDK):

uint8_t buffer[UDI_VENDOR_EPS_SIZE_ISO_HS]; // 64 bytes.for (int i =
0; i < sizeof(buffer); ++i)
    buffer[i] = i;for (;;){
    led = !led;
    udi_vendor_iso_in_run(buffer, sizeof(buffer), nullptr);
    // TODO: How to wait until it is finished?
    buffer[0]++;
    if (buffer[0] == 0)
        buffer[1]++;}

The receiving program is basically the example iso read program from
libusbk. It writes data to a log file, including the first two bytes of
each packet which I increment as shown above. It requests 128 packets per
frame, and outputs data like this:

#6: StartFrame=00DDE360h LastFrameCount=16 TransferLength=8192
BPS-average:390409.30
  #0000: Length=64 C0h 3Dh
  #0001: Length=64 E1h 3Dh
  #0002: Length=64 01h 3Eh
  #0003: Length=64 23h 3Eh
  #0004: Length=64 44h 3Eh
  #0005: Length=64 64h 3Eh
  #0006: Length=64 85h 3Eh
  #0007: Length=64 A6h 3Eh
  #0008: Length=64 C7h 3Eh
 ... snip ...
  #0120: Length=64 34h 4Dh
  #0121: Length=64 55h 4Dh
  #0122: Length=64 76h 4Dh
  #0123: Length=64 97h 4Dh
  #0124: Length=64 B8h 4Dh
  #0125: Length=64 D9h 4Dh
  #0126: Length=64 FAh 4Dh
  #0127: Length=64 1Bh 4Eh

As you can see, it works but is skipping loads of the packets. I'd like to
know why! The data rate is really low compared to what USB 2 can do. I'm
kind of at a loss here as I don't know how to wait until a transfer is
completed in a tight loop on the SAM3X. Also... it should work right?

Here are the USB descriptors (I snipped some unused endpoints):

Device Power State:               PowerDeviceD0

       ---===>Device Information<===---English product name: ""
ConnectionStatus:                  Current Config Value:
0x01  -> Device Bus Speed: HighDevice Address:
0x05Open Pipes:                           0*!*ERROR:  No open pipes!

          ===>Device Descriptor<===
bLength:                           0x12
bDescriptorType:                   0x01
bcdUSB:                          0x0200
bDeviceClass:                      0x00  -> This is an Interface Class
Defined Device
bDeviceSubClass:                   0x00
bDeviceProtocol:                   0x00
bMaxPacketSize0:                   0x40 = (64) Bytes
idVendor:                        0x1111 = Dade Behring, Inc.  // Screw
the USB-IF!
idProduct:                       0x2223
bcdDevice:                       0x0100
iManufacturer:                     0x01
     English (United States)  ""
iProduct:                          0x02
     English (United States)  ""
iSerialNumber:                     0x00
bNumConfigurations:                0x01

       ---===>Full Configuration Descriptor<===---

          ===>Configuration Descriptor<===
bLength:                           0x09
bDescriptorType:                   0x02
wTotalLength:                    0x0045  -> Validated
bNumInterfaces:                    0x01
bConfigurationValue:               0x01
iConfiguration:                    0x00
bmAttributes:                      0x80  -> Bus PoweredMaxPower:
                   0x32 = 100 mA

          ===>Interface Descriptor<===
bLength:                           0x09
bDescriptorType:                   0x04
bInterfaceNumber:                  0x00
bAlternateSetting:                 0x00
bNumEndpoints:                     0x00
bInterfaceClass:                   0xFF  -> Interface Class Unknown to USBView
bInterfaceSubClass:                0xFF
bInterfaceProtocol:                0xFF
iInterface:                        0x00

          ===>Interface Descriptor<===
bLength:                           0x09
bDescriptorType:                   0x04
bInterfaceNumber:                  0x00
bAlternateSetting:                 0x01
bNumEndpoints:                     0x06
bInterfaceClass:                   0xFF  -> Interface Class Unknown to USBView
bInterfaceSubClass:                0xFF
bInterfaceProtocol:                0xFF
iInterface:                        0x00
(snipped irrelevant endpoints)

          ===>Endpoint Descriptor<===       <---- This is the endpoint
I'm using.
bLength:                           0x07
bDescriptorType:                   0x05
bEndpointAddress:                  0x81  -> Direction: IN - EndpointID: 1
bmAttributes:                      0x01  -> Isochronous Transfer Type,
Synchronization Type = No Synchronization, Usage Type = Data Endpoint
wMaxPacketSize:                  0x0040 = 1 transactions per
microframe, 0x40 max bytes
bInterval:                         0x01

Does anyone have any ideas?

Cheers,

Tim

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