From my occasional forays into the forum for advice I have never been disappointed that someone out there has something useful to say.
On this occasion I am hoping someone knowledgeable and experienced will save me having to dismantle a tunnel to get at a point to lift it for examination and repair or replacement.
I am surmising that when laying the track I did not adequately solder my green frog wire to the Peco electrofrog point's frog wire. All locos will make the exit when the point is set out, but when straight on the smaller jinty style locos inevitably stall. Those locos with longer wheel bases more often than not negotiate the point in that direction, but not always. It is controlled by a Cobalt IP. 
Is there something within the Cobalt's internal switches which may cause the difficulty? I have a spare Cobalt, and replacing the motor would be much easier than lifting the point.
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Is it possible to check for continuity between the frog itself and the end of the supply wire?  Disconnect the frog supply wire from the cobalt switch. If you can somehow get a meter lead onto the frog you should be able to prove that you have good continuity or not.

Again just simple continuity, if you have continuity to the frog then check the switch changes over from one side of the supply to the other when the point changes.

Just do as much continuity testing as possible to start with to try to see if everything is as it should be.  This might require you to power up, change the position then power off again but do as much as you can without power to start with.
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IP Analog or IP Digital? It's actually exceptionally rare for a switch on an iP-Series Cobalt to fail as its a seriously robust system.

Did you bond the point closure rails to the left and right stock rails? If not you likely have a long section of dead rail rather than just a dead frog - or has the BLADE lost contact with the closure rail? Its a pretty fragile contact in the first place. Those things wouldalso  explain why larger locos are stopping too.

AND... pickups often do not actually touch all the wheels unless tweaked... no matter how clean they are, as there is so much lateral slop in the wheelsets don't underestimate this as a factor.

If you are using a separate switch in an IP Analog (using left/right and common to the frog), then did you definitely strip enough (as per our instructions) and insert far enough into both the L and R - if a wire is short-stripped it may seem to be "in" but not be properly making contact within the terminal.

As BB said, lots of logical testing. I suspect however its the point not the switch
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Thanks for the input. Valuable as usual. I agree with Ahjay's conclusion that the problem is unlikely to reside within the Cobalt IP Digital but I will thoroughly check for short stripped leads before I resort to lifting the point for final inspection and repair,
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I too have a similar problem with short wheelbase 0-6-0 (Farish) stalling. I have a GWR ‘flying banana’ which negotiates the points at almost zero speed, so it is not the fault of the points. Stripping down the Farish reveals probably too much lubrication surrounding the bearings, but more importantly too much VERTICAL play in the bearings, so that the connecting rods can actually prevent wheels dropping onto the rails, so losing connectivity. It was especially noticeable when using the DCC Concepts rolling road and looking at the roller bearings when the loco was under power. In a lot of instances, movement of the rollers was sporadic at best, and it certainly looked like vertical play and connecting rods are the culprits. Not much I can do about it, as the ideal solution would be a compensated chassis, but N gauge is a bit small for that
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