Aptimal 1 not dissolving properly? (also in Baby)

Hi folks our wee one has been FF on Aptimal 1 for about 4 weeks as I had to give up BF due to illness (mine and hers were both in hospital and just couldn't continue) - she has been dead happy on it but the past few days got a bit fussy, and started to split her 5/6 oz feeds into 2 or 3 oz but more often - she seems happier to take the cartons, and as an experiment we;ve used the cartons all today and she's back to straight 5 or 6 oz feeds every 3-4 hours - just wondering, hubby and I have both had problems getting our latest box of Aptimal powder to fully dissolve - milk doesn't LOOK lumpy but there seems to be more bits of powder on the side of the bottle once she's drained it than before and we haven't changed the way we're making it! Anyone had dodgy boxes before, or any tips? We make it with water that's cooled for no longer than 15 minutes as we found that waiting 30 meant it never mixed well, and then let it cool down or run under the tap.
any advice appreciated! can't afford to feed her on cartons all the time, we'll bankrupt ourselves! (though I can't say I blame her, the stuff in the cartons always looks better to me as well!)
xx