JamMan records loop but not with the effects from Marshall AS100, Ideas?

Your effect loop is fried.

NEVER CONNECT PEDALS THROUGH THE EFFECT LOOP OF AN AMP. They are NOT design to work at the same signal levels !!! ONLY use effect units that are design to be used in amp effects in the amp effects loop/

That is the way it is supposed to work... It is a level control to control the level sent to an EXTERNAL effects unit. What external effects unit are you using? Brand and model please. If you proportion more to the effects loop, you want to make sure the EXTERNAL effects unit is on and its gain controls are up. The foot switch input should control the EXTERNAL effects unit when you plug the footswitch into the EXTERNAL unit. The footswitch input on the amp itself has NOTHING to do with the external effect loop, it only affects the amp's own DFX unit.

You might be using an effects loop with no gain adjust or similar.

Read your manuals for both pieces of gear carefully first. You only have one amp so you won't be able to take advantage of all the FX on the Digitech as a lot of them are designed for stereo. The Digitech manual shows guitar into the Digitech and output from the Digitech into input of any guitar amp. Since you have a Marshall and can already get some great sounds by running your guitar straight into the Marshalls input, you would probably be better off using your Digitech in the FX loop. Run a guitar cord out of the send on the loop into the input of the Digitech and a guitar cord out of the left / mono output jack on the Digitech into the effects return jack on the Marshall. Start out with the effects level on the Marshall all the way down. Do the same with the Digitech with both the input and output levels. (Read the manual carefully) I think the choice of the Digitech isn't that great because it is already trying to reproduce a lot of what you can already get with just your Marshall but with a little experimentation and some careful adjustments you should be able to add to the sound of the Marshall. Hope this helps.

Usually the loop in is used with an EXTERNAL effects module where the audio signal is "looped" out through the external effect module and then back to the power amp to drive the speaker in the cabinet. The LOOP IN jack would receive signal from the EFFECTS module OUT jack.

The source to go TO the effects module INPUT may come from a jack such as preamp out, or sometimes the same loop jack if it usess the TRS (Tip-Ring-Sleeve) connection method.

If you do not have an external effects module, all this is a moot point in your case.

Sometimes the sound can feedback onto the strings of the guitar effectively sustaining.

Also check it with reverb off. Sometimes the effects loop and create sustain.

Well if there is anything you need to insert into the effects loop IE, wah pedals, noise gates etc etc the return is the output of the effect and the send would go to the input of the effect. As far as inputs those are all on the front panel.

This sounds like a thermal problem. You can often find the component by spraying suspected ones with circuit cooler, looking for a change.

If you only connected the peaker to the wrong connector USUALLY no damage was done. SOMETIMES the effects loop jack which has a switch contact on it MAY not connect again when you removed the speaker connector. Some of these have very weak jacks Try putting a cable into the jack several times and removing to cycle the contact to possibly clean it. The other possibility is if you didn't have a speaker load you might have popped the power amplifier... hope not...
You MAY have to take this to a shop to repair this.