More lemon juice is more liquid, which is likely the cause of your problems. If there's any other liquid in the recipe, you can try swapping it for the lemon juice, but it's possible that it might provide something else to the recipe that lemon juice can't provide.

You can try adding something else that's sour (tamarind, sumac, etc), or specifically buying powdered citric acid.

I would also recommend zesting the lemon (only the yellow part, avoid the bitter white pith) to help increase the lemon flavor without adding more liquid. You can also buy food-grade lemon oil or lemon extract.

A few suggestions:

• Add lemon zest as well as the juice; this is often a better way to convey lemon flavour. I would mix it with the sugar a little while before using it to help extract the essential oils, and maybe sprinkle some zest on top so that it contributes to the smell when the pie is being eaten.
• Reduce your greater amount of lemon juice to the right volume of liquid by heating it for a while in a wide-bottomed saucepan. I can imagine this will affect the flavour but it shouldn't reduce the sourness if that's what you're going for.
• Experiment with reducing the amount of sugar, which does affect perceived sweetness. However, this could also have an effect on the general structure of your pie (and I assume you still want it sweet).
• Add lemon essence to the recipe, much like you might use vanilla essence in a sponge cake.

One possibility would be to add lemon zest (for lemon flavour) and then lime juice (rather than lemon juice) - or a mixture of lime and lemon juice. Lime is more acidic than lemon.

If you don't mind a slightly artificial flavour, you could (as others have suggested) add citric acid - dissolve the powder in around the same volume of liquid (in this case perhaps in lemon juice). There are also bartender's products like supasawa which will do the same thing and allegedly taste less artificial due to the use of multiple acids.

One problem you might be having is that the acid is preventing the setting of the pie. I doubt this is the issue as many such recipes rely on the acidity (through curdling). But if so, perhaps look for tarte au citron recipes that work well with acid.

Much depends on the rest of your recipe, but you could experiment with Amchur/Amchoor, dried and powdered unripe mango. It is fruity but sour and is also used for its thickening properties:

As useful and wonderful as lemons and limes are to livening up long-cooked dishes, they can get a little old. Amchoor has the tartness of citrus and all the flavor of its zest. Like citrus juice, it's best added at the end of cooking to preserve its flavor, but should be stirred well into whatever its coating. Similarly, it can also cut the cloying sweetness of fruit-based sauces, syrups, and compotes. It makes an interesting addition or alternative to the medieval triad of cinnamon, clove, and nutmeg.

It has one other advantage over lemons and limes: It adds no liquid. If you want to add tartness to some roasted vegetables or fried items while preserving their delicate crispiness for more than a few minutes, amchoor may be your answer.

You might need to be careful in case it adds a graininess, possibly like cocoa powder you need to be careful to make sure it is fully hydrated before you bake, in which case soaking it in the lemon juice might work well.

You can try adding concentrated lemonade. That will increase the sweetness, so you should reduce the sugar elsewhere. You can also try adding something with a very low pH. For instance, if you can get your hands on some citric acid, you could try that.

Citric Acid

This is exactly the same compound that makes lemons, and all citrus fruit, sour. Adding more will allow you to adjust the sourness without changing anything else in the recipe.

60ml (2oz, 4 tablespoons) of lemon juice typically has about 3 grams of citric acid and is about as much juice as you get from a lemon, so to double the sourness you would add about 3 grams of citric acid per medium lemon's worth of sourness. 3 grams should work out to just over half a teaspoon, if that's all you have to measure with.

Citric acid is both cheap and readily available - be sure to get food grade.

A few other answers have touched on this as a passing idea, but it's absolutely the way to go, 100%. Other souring agents like sumac or tamarind will ruin the lemon flavour - they have way too much of their own personality and will completely throw the aromas off. Zest and lemon oil will give you a more powerful lemon aroma, but will do nothing for the sourness.