I have noticed that I get multiple sync failures after doing log dives (90 minutes and greater).  It appears the FIT file gets to a size where a timeout between the Descent and Garmin Connect Mobile becomes more likely.  This happens even when the Descent is less than a metre from my iPhone.
I'm sorry the image is so small.  These fora make it impossible to upload a image and keep its size.  You can at least see how many times sync has failed.
Here are the errors from the sync report which are common across all the failed syncs for long dives.  Eventually, it will sync but it can take an hour of trying.
[FONT=courier new]UPLOADS
   File Type           Filename                                        Size (bytes)   Status      
   -------------------------------------------------------------------------------------------------
   FIT_TYPE_4          1-3966683484ble-FIT_TYPE_4-2859-31-893653287    245543         Error       
   FIT_TYPE_35         (null)79             Cancelled   
   FIT_TYPE_32         (null)2901           Cancelled   
   FIT_TYPE_32         (null)484            Cancelled   
   FIT_TYPE_32         (null)431            Cancelled   
   FIT_TYPE_32         (null)624            Cancelled   
   FIT_TYPE_32         (null)431            Cancelled   
   FIT_TYPE_32         (null)569            Cancelled   
   FIT_TYPE_32         (null)554            Cancelled   
   FIT_TYPE_32         (null)902            Cancelled   
   FIT_TYPE_32         (null)749            Cancelled   
   FIT_TYPE_32         (null)554            Cancelled   
   FIT_TYPE_44         (null)335            Cancelled   
   FIT_TYPE_41         (null)14789          Cancelled   
DOWNLOAD DEVICE FILE TYPES
   File Type           
   -------------------------------------------------------------------------------------------------
DOWNLOADS
   File Type           Download ID         Size (bytes)        
   -------------------------------------------------------------------------------------------------
ERROR DETAILS
Error Domain=GCMSyncErrorDomain Code=2 "Device connection error" UserInfo={SyncErrorReason=File UL: Failed to receive file from device, NSLocalizedDescription=Device connection error, NSUnderlyingError=0x1c4a5f3e0 {Error Domain=GarminDeviceSync.FileTransferErrorDomain Code=16 "(null)" UserInfo={GarminDeviceMessageError.ErrorReasonKey=Timed out waiting for file data request.}}}[/FONT]
								
				
				
		 
				 
		 
					 
				