We are using aptamil anti-reflux. How are you making it up? Unlike other formulas it can't be made up in very hot water - I think this might be what is causing your clumps. We had the same issue at the beginning. The instructions on the packet say to use hand-hot water. We've found this (or body temperature water) works fine although you do have to really shake it. There are lots of pitfalls with this formula (the fact you can't use very hot water, the serious shaking needed, the fact that even when it is mixed well about a centimetre of foam is left in the bottle even when it is 'empty') but for us it has been a godsend as my little one is no longer throwing up everywhere and is starting to gain some proper weight. Hurrah. We've had lots of teething problems and have figured things out through trial and error (and google!) so happy to answer any other questions you've got.