The highest and lowest reeds tend to be problematic because they are made to a scale length that is practical, rather than ideal - otherwise the high reeds would be too small to be manageable, and the low ones too long, necessitating an oversized instrument to accomodate them. Hence the high reeds are filed very thin towards the tip, to raise the pitch, whilst the low ones are thinner at the root, or even with weighted tips, to lower them.
But, as a result of this, the high reeds need special treatment, including less air pressure, and various "tricks" are used, to try to get them to speak well. This is why you'll find they don't have valves on them, and sometimes there are venting holes in the chamber walls too - usually through the wax - to lower the pressure on them.