Distance of Rocket M2 with Airmax Sector

We recently setup a new 3 sector array at 190ft on one of our towers. We used Ubiquiti Rocket M2's with Ubiquiti 120 15db Sectors. The terrain is primarily flat with a few hills. At this point we are only able to achieve about 5 mile connections with Bullet M2HP and 19 Parabolic antenna attached to our service truck. Anything over 5 miles and the signal drops down to unusable. We have changed the downtilt from 4deg to 0deg and that didn't change the distance. We have worked with the firmware and are currently running version 5.2.1. When we are testing we can see the tower at about 7 miles and get signals in the 80s.
We currently have 8 other towers that we run mikrotik routerboards with valemount cards on 16db sectors and can achieve 8 - 10 mile links. I am just wondering is this a limitation of the equipment or am I doing something wrong? 

the UBNT antenna's already have a 2 degree down tilt so it should be good at 0 you should be able to link as far as you can see the antenna Please post your config file Also if your trying to use a Bullet with AirMax ON in the AP I could see Problems with that 

I have 2 GHz Rocket M2's on 16 dB sectors 200 feet high. With NSM2's, I've been able to get about 3-4 miles. I turned off airmax in the AP's and am now able to get 7-8 miles with Airgrid M2's on a pushpole. Airgrid M2's won't do MIMO. Only NSM 2's right? So what does MIMO get me? More throughput, anything else? Is there a long distance airmax compatible/MIMO CPE, or is it too hard to build cost effective antennas for MIMO/Airmax?
Is that pretty much my choice? Airmax or long distance client connections? Can't have both? 

I can't see why Airmax would limit the distance to 3-4 miles, we have 14 mile links in the 5gig band with Airmax on. Course it's 30 dish rocket combo to a 15 120* sector rocket, but still Airmax is on.  

You can use the high gain (18dBi) NanoBridge as a MIMO CPE

For long-distance 2X2 MIMO links, use
NanoBridge. It supports AirMax of course.  

Airmax isn't what's limiting the distance, it's my unfamiliarity with the product. I've never tried the Nanobridge, didn't know to try it. I got the Airgrid because it has a dish, the reflectors aren't supported for NSM's, I figured a grid dish would get me alot further from the sector, so I tried it.
Nanobridge has a solid dish? How does that work on a push pole? I don't worry too much about a grid dish on a pushpole, but a solid dish? I don't know......

NanoBridge will have more wind
resistance than AirGrid.

I will post my config in the morning, but what are you talking about when you state that if I am using bullet with airmax on? Are you talking the straight bullet or are you talking about Bulletm2HP? Bullet M2HP is designed for the airmax! Thank You for your time and I will have the config in the morning.  

What do you have your powers set at on your ap and what types of signals do you have at the airgrid? How many sectors do you have up the tower? Thank you very much.