Some tubes do sometimes display less 'illumination' than other identical tubes. Were the tubes matched when you bought them? As the amp is auto-bias it really shouldn't be much of an issue. If it sounds okay I would think all is well.
I mean if it's only barely lit compared to others being bright I'd think again but if it just seems a bit 'dull' in comparison it's unlikely an actual issue.
You could also swap the non illuminated tube for one that does illuminate more in another socket, then you can determine if it is the tube or the amp. Again if it is auto bias and sounds okay, I wouldn't worry too much. Also if they are NOS tubes and haven't been used before they will change a bit (as I'm sure you know). I'd leave it on for a couple of hours and have a listen, any loud crackling/imbalance and I'd switch off and have a rethink.
Been a while since I had one but I loved my old EL84 amplifier. Very sweet top end and just a fun sound.