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In that version, the slide effect (when holding fingers across notes, like A to D) would trigger based on how hard you played – it responded to velocity.

We don't have  Modx/Modx+ but let me describe what I found on our Montage M8X and maybe you can see if that agrees with what you have.

On our Montage M8x there is a performance named 'Nylon Slide Vel'. It has one part with 8 elements.

Pressing a key lightly or moderately produces one tone. Pressing the same key hard produces a 'slide' from 1/2 tone lower to the tone of the key pressed.

Could that be the 'slide' you refer to? Test using just ONE note to see rather than 'A to D'.

On the M8X Element 1 is defined with an 'Elem Vel Limit' of 1-120 but element 2 has a limit range of 121-127.

So pressing a key hard triggers element 2 which uses the 'Nylon1 Slide St' waveform 619 on the M8X. That is listed as waveform 589 in the Modxk+ Data List doc so on your instrument it is likely the same performance and using the same waveform.

Try the test above and, if you need to, examine elements 1 and 2 to see if they are using the same waveforms.

If I use an Init Normal (AWM2) performance and replace the default waveform with #619 'Nylon1 Slide St' I get the same 'slide' effect mentioned above. 

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The above should address your issue but as an FYI we should mention that, at times, there can be other 'GOTCHA' things going on.

That performance has the part Portamento switch is set to ON. But the Portamento Master Switch is set to OFF by default on the M8X. Both the Master and Part Portamento switches need to be ON for Portamento to operate.

On the M8X that preset performance has the Master Portamento switch set to OFF but I don't know if the original Modx had it set to ON or not or if the Modxk+ has it set to On or Off.

With the Portamento Master Sw to ON you will get a 'slide' between two DIFFERENT notes rather than on a single note. But there is no velocity involved here.

And if you examine the other 6 elements you will find that some of them operate based on the settings of  one of the assignable switches.

By default (M8X) those switches are set to 'Momentary' for that performance so you would have to manually press them to trigger those other elements.

But if you had changed the setting to 'Latch' and turned an assignable switch ON then it would stay on and would only trigger those higher number elements. That would definitely sound differently since the real elements (1 and 2) won't sound if the switches are on.

Please post back and let us know if you resolved the problem.

The MODX8's heavier keybed may be more difficult for you to reach velocity levels required to produce the slide.    First, try just forcing the velocity to be 127 to sort of prove that point:

1) Press [UTILITY] button

2) Navigate to "Settings" -> "Sound" menu.

3) Set "Velocity Curve" to "Fixed" and then set the fixed value to 127.

... now playing the “Nylon Slide Vel. Guitar.” should create a slide for every note you play.   No matter how hard you strike the key the velocity will always be high enough to generate the slide sound.

If that does produce the slide sound then now it's a matter of perhaps adjusting the velocity curve to make it easier for you to reach the required velocity.  Instead of "fixed" try "soft".  Although I have to say that it's unlikely the [UTILITY] (settings) curve will help at the extremes needed to produce the slide.  Instead, I would probably go into the elements and dial down the velocity lower limit for the slide sounds so a lower velocity threshold is needed to generate the slide effect.  Either that or get more ham fisted.   If you modify the velocity lower limit then make sure to [STORE] the Performance as a User Performance and name it appropriately so you can find (and use) the modified Performance.

If that (fixing the velocity to 127) doesn't produce the slide sound then something else is going on.