Antenna Analyser Frustration

I'm in the process of installing a sloper antenna for 20M.  Summer is clearly on the way out and I want to get this done sooner rather than later as we all know how easy it is to find ourselves doing antenna work in November.  I want to cut this sloper so its resonant on 14.06 for QRP use with my RockMite.  I have the MFJ-207 Antenna Analyser and, if you are unfamiliar with it, it has 5 range settings from A to E.  I use the analyser with a frequency counter so I know exactly what the frequency is when I am reading resonance.  The top end of range D, which is supposed to be 14, seems to end at around 13.5 MHz and the bottom end of range E, which is also supposed to be 14, comes in at around 14.5 MHz.  I am not able to tune the antenna where I want to.  I'm pretty sure this is not by design as the 20M band is pretty popular.  I don't know if this is just a bad unit I purchased but I am definitely not pleased.  I have contacted them via their web form and hopefully I will hear back before too long.  I haven't searched the internet to see if this is a common problem with these units.  I guess that's my next step.