what was the conditions like what is flat calm if so you need to go deeper they will only feed shallow with confidence if there is a ripplel on the water the more the ripple the more confidence they will get the more calmer it is the less confident they get try that hope it helps

How long was your hair? For F1s I like the pellet to be almost touching the bend of the hook. Also for a 4MM pellet I’d not go above an 18, and don’t settle for anything less than a perfectly sharp hook (I use size 18 Middy KM2s, which are strong enough for bonus mirrors & commons but not too heavy).

Difficult to know if switching to an 18 would have made any difference, there are other things that could have helped too. e.g. Feeding, maybe a bit less or less but more often would have got them competing for pellets more, competing fish don’t have the time to be careful biters… Depth can make a huge difference & getting it right can make the difference between missing them and laccy streaking out before you’ve had time to react. If there’s no rule for minimum distance from pole tip to float, & conditions allow, it can help to have less than 6″ of lash – it speeds up the strike & some will self hook.

Sounds like you might have been over feeding and whipping them into a frenzy. Once they are boiling on the surface you need to cut down the feed t odrop them back down a bit or you will just get hundreds of liners as you describe (i don’t think they were shy bites).

I think the hair is too short, bait gets in the way of the hook, should be about a pellet’s distance away. Also try feeding a lot less as said and try thicker or thinner line, thicker is often better!