U-Boot bootcmd (auto vs manual)

I'm running into an issue where when I power up my device, it hangs at

Starting kernel ...

Or it loops with

Starting kernel ...

resetting ...

However, if I interrupt the boot process and manually run boot, ie:

=> run bootcmd
or
=> boot

then the kernel loads fine.

According to DENX (5.9.6.5) this is equivalent to what Uboot should be doing automatically.

Does anyone know if there is a difference between letting uboot run on it's own and interrupting and running boot manually? Otherwise, how do I start debugging this?

ENVIRONMENT

=> printenv
autoload=no
baudrate=115200
board_name=EVK
board_rev=pilot
boot_fdt=try
bootcmd=echo Booting from network ...; usb start; setenv ethact asx0; if dhcp && tftp $loadaddr $bootfile && tftp $f
dt_addr $fdt_file; then run nfsboot; else echo WARN: Issue with TFTP.; run sdboot; fi;
bootdelay=3
bootfile=zImage
bootscript=echo Running bootscript from mmc ...; source
console=ttymxc1
ethact=asx0
ethprime=FEC
fdt_addr=0x83000000
fdt_file=imx6ul-pilot-v1-evk.dtb
fdt_high=0xffffffff
image=zImage
initrd_high=0xffffffff
ip_dyn=yes
loadaddr=0x80800000
loadbootscript=fatload mmc ${mmcdev}:${mmcpart} ${loadaddr} ${script};
loadfdt=fatload mmc ${mmcdev}:${mmcpart} ${fdt_addr} ${fdt_file}
loadimage=fatload mmc ${mmcdev}:${mmcpart} ${loadaddr} ${image}
mmcargs=setenv bootargs console=${console},${baudrate} root=${mmcroot}
mmcautodetect=yes
mmcboot=echo Booting from mmc ...; run mmcargs; if test ${boot_fdt} = yes || test ${boot_fdt} = try; then if run loadfdt; then bootz ${loadaddr} - ${fdt_addr}; else if test ${boot_fdt} = try; then bootz; else echo WARN: Cannot load the DT; fi; fi; else bootz; fi;
mmcdev=1
mmcpart=1
mmcroot=/dev/mmcblk1p2 rootwait rw
netargs=setenv bootargs console=${console},${baudrate} root=/dev/nfs ip=dhcp nfsroot=${serverip}:${nfsroot},v3,tcp
nfsboot=run netargs; bootz $loadaddr - $fdt_addr
nfsroot=/nfs/rootfs
script=boot.scr
sdboot=echo Booting from mmc ...; mmc dev ${mmcdev}; mmc dev ${mmcdev}; if mmc rescan; then if run loadbootscript; then run bootscript; else if run loadimage; then run mmcboot; else echo ERROR: Cannot run loadimage; fi; fi; else run ERROR: Cannot run mmc rescan; fi;
serverip=192.168.0.219

Environment size: 1714/8188 bytes

This issue was with the SDBOOT env var:

sdboot = echo Booting from mmc ...; mmc dev ${mmcdev}; mmc dev ${mmcdev}; 
         if mmc rescan; then 
             if run loadbootscript; then 
                 run bootscript; 
             else 
                 if run loadimage; then 
                     run mmcboot; 
                 else 
                     echo ERROR: Cannot run loadimage; 
                 fi; 
             fi; 
         else 
             echo ERROR: Cannot run mmc rescan; 
         fi;

Load bootscript kept failing with "** Unable to read file boot.scr **". I'm still not sure what was causing the difference but removing the loadbootscript branch and going straight to loadimage fixed the issue.