According to the table in the Dexter Service Manual PDF (See below for URL), your two 7" magnets should be pulling 5 Amps -- Using the P3's very handy troubleshooting mode [Setup>Help>Troubleshoot>OK], you can get the current output right from the screen of the controller -- Clearly, if you are seeing only 2.5A, one of the magnets is not functioning and if a reading higher than 5A is seen, there is a short somewhere.
I currently have a Prodigy, but if I were changing it out for some reason, I'd likely get the P3 because of the Current feature -- It would only be about $20 more on EBay. BTW, don't sweat buying used from EBay (bidding on a used Prodigy is at only $26 on one right now) because both the Prodigy and P3 have Limited Lifetime Warranty -- Just take the bad unit into a dealer and they will swap it for a good unit.
I retrofitted 7"x1 1/4" brakes on my Dexter 2,200 lb #9 axle, with the loaded trailer at a scale weight
of 1,700 lbs and am able to lock up the brakes per the controller instructions -- They are good for up to 2,200 lbs per pair.
You have the right brakes on the right axle for your Boler
13' -- Ten inch brakes start on the #10 axle (rubberable up to 3,500 lbs) typically used on the 16' trailers.
The P3 instructions (PDF download from Tekonsha) had this note regarding Lock-Up:
"2. WARNING The power should never be set high
enough to cause trailer brakes to lock up. Skidding
trailer wheels can cause loss of directional stability
of trailer and tow vehicle.
3. The power/Boost may need to be adjusted for
different load weights and road conditions.
4. Not all trailer brakes will lock up due to various
conditions. However, inability to lock up the
brakes generally indicates the need for an
inspection to determine the cause."
Here's the URL for the Dexter PDF for the parts breakout for the various flavors of 7" brakes; a similar PDF is available for the 10" brakes:
Note: I believe the two pages of 7"x1 1/4" brakes with 600-1100 lbs marked on the page edge are for the smaller #8 axle used on utility trailers, etc. -- I don't see any difference in parts numbers between the two sets and both are rated for the current axle maximum of 2,200 lbs.
Sadly, the self-adjusting feature is only available in the 12" brakes for larger trailers; the Nev-R-Lube cartridge bearing is only available on the #10 and up axles, with six-lug hubs and wheels (It would be really nice to have those bearings and not have to mess with grease again, just monitor for heat!).
Lots of good axle information on the Dexter site (www.dexteraxle.com
); well worth the digging and down-loading.
Here's Dexter's Service Manual PDF for electric brakes, including troubleshooting:
Once you have them working, I would seat the brakes and then redo the controller adjustments.
Dexter sez to seat the brakes by applying them 20-30 times at initial 40 mph, slowing to 20 mph and allowing sufficient time for brakes to cool between applications.
Additionally, Tekonsha sez:
"Always warm the trailer's brakes before setting the
power. Warm trailer brakes tend to be more
responsive than cold brakes. To warm trailer
brakes, drive a short distance (1/4 mile) at 45 MPH
with manual lever engaged enough to cause trailer
braking at a low level."
If all else fails, you might try pulling the trailer down a dirt or gravel road and locking the brakes with the controller lever, then examining your tracks on the road -- One side may not be working -- Be careful because if only one side is working, the trailer may try to turn the tow vehicle away from that side.