I bought four potato bags last year, planted up (late) and they came to nothing. Didn't get a single potato that was big enough to bother with - and not many of those. The instructions said plant up with compost, cover, add more as the haulms show through.
This year I've done it differently: I planted up the bags with a mix of
some compost but mostly soil from the veg plot (manured last year). I put the potatoes in and then filled the bags right up. I water every few days and already the haulms are starting to show through.
Can't say how many I'll get this year, but can't be worse than last!