Bells of Ireland is considered a cool season plant. Lavatera trimestris is not supposed to be, but when I planted it last year, it died out by midsummer. One person's one time experience isn't definitive, but maybe others will chime in.

I find that there are a lot of annuals that can't take frost, but can't take the heat either. They probably do very well as summer annuals in cool climates, but I have given up on ever growing these types. I find Nasturiums, Nemesia, Salpiglossis, Schizanthus and Brachycome are like this too.

Lavatera are BEAUTIFUL when in full flower! I have grown them in the past, and I found that after the first flowering, which occured around beginning of July and lasted about a month, they petered out and didn't really flower much, if at all, the rest of the season. I grew them a couple seasons in a row, and each experience was the same. Mine were grown from seed each season.

I don't grow them anymore because I plant annuals for sheer flower power - if I wanted something in a particular spot with only a month or so of solid blooming, I would plant a perennial.

You could do succession plantings, sow three to four batches of seeds about a month apart, and that should solve the problem.