I installed a LWD 3.5# connecter and 6# spring on a Gen4 G19. After reassembly I noticed the trigger wouldn't fully reset, I took it back apart and reinstalled the stock trigger spring and the trigger reset perfectly.

Am I missing something here? Do I also need to replace the striker spring when replacing the trigger spring?

I think what you are missing is that aftermarket parts are causing problems.  I've been shooting Glocks for close to 20 years and have never had a problem with a stock gun.  I've seen many problems with guns containing aftermarket parts.

If the 6# spring you're referring to is the striker/firing pin spring, then yes this is a known issue (especially in .45/10mm guns). Actually, if you read the instructions (I think in the Ghost trigger kit) they only recommend using the light striker spring in 9mm/.40 Glocks for COMPETITION ONLY and do NOT recommend using them in the .45/10mm guns at all. I bought a used G21 that suffered this issue and I remedied it by replacing with the correct striker spring.

The reason is that the striker spring is so light that when it catches the trigger bar it does not have enough force to move the trigger all the way forward and overcome the trigger spring, instead the striker just moves rearward towards the "cocked" position.

Edit: Just re-read your post. You say replaced trigger spring and therefore I'm not sure other than an out of spec part or install error. Can you post pics? I run mine with just the trigger spring or trigger spring and connector and have not had any issues. I'm going to leave my initial response intact for others that may have striker spring related issues.  

The Glock trigger spring actually helps pull the trigger; therefore, a heavier trigger spring is going to be a detriment to the trigger resetting.  I'm no expert, but I have read and heard directly from people who are that the trigger spring should NOT be changed on a Gen 4.

Also, make sure you have the spring installed in the correct manner. Having the spring "coiled" or opposite of the photo below has been known to cause issues. Gen3 or Gen4 gun?

Trigger spring orientation

I run these springs on all my Glocks Gen 2,3,4 and many different models including 21,19, 32, 34, 17 never had an issue.  But you must be sure you install the spring the same way the one you removed.  
The spring should aid in reset making it more positive and a lighter trigger pull.

Like previous poster posted make sure the spring is installed correctly.

I will check again but I am positive that the LWD spring was in that orientation and the stock spring is as well.

This is the orientation of the stock spring.

I cannot tell fully but it looks like the trigger bar is not fully seated on the connector.

That *should* straighten itself out once the slide is back on. You can move the trigger bar over when the slide is off (sometimes happens during slide removal) and this is the result. I don't believe this should have anything to do with his issue.

I'm not familiar with LWD, is this the connector where you need to file-fit the little tab to achieve the desired reset?

Trigger spring looks good on the top end, check to make sure the open end on the bottom is facing correct way.

I have been using the acronym but it's Lone Wolf Distributors, I hadn't read anything about needing to file it.

I just meant I've never used one of their connectors.

I have used/use LWD, Glock, & Scherer with best feel going to Scherer but never a reset problem with any amongst multi gen and different frames.

Sounds like some trial and error time.  If you are assembling it correctly try it with just the connector swapped then also try it with just the heavier reset spring swapped see which give problem.

The LWD connecter and Glock spring works fine. I just tried the Glock connecter and LWD spring and that still gave me the reset issues.

Could be a bad spring.  See if the seller will exchange.  I recommend Zev springs for this.

I've run into this same problem a couple of times. All were with Gen4 models. Put the stock trigger spring back in and shoot a couple of hundred rounds through it and try the heavier spring again and it may work. I've got a Gen4 G17 that hates the heavier trigger spring so I just have the 3.5 lb connector, lighter striker spring, and lighter safety plunger spring installed and I still get a nice trigger...this is a competition pistol.