Here's a photo of the petunia

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What kind of potting medium are you using?

Promix.

I've tried several other types of potting mix and they all seem to have the same problem. I've used Miracle grow potting mix with the same results.

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Thanks. But we need a few more clues.

We need to know what mix you were using when the problem began. And how long the plant was in it.
Then, what was the next mix, and how long the plant was in that.
And the next mix, and so on until we get to today and the mix it's in now.

Also, you say you are using "weak solution of fertilizer applied once a week." So we need more info:
What fertilizer do you use?
What is the suggested dose? (like teaspoons per gallon)
And what is your "weak solution?" (again, the # teaspoons per gallon)

I can think of a couple of things that could cause that. I think somehow the nutrients are not getting to your plants.

Those are pretty big plants. What size pot do you have them in? If the pots are too small the plants could be root bound--hence the roots can't take up the fertilizer.

How do you water and fertilize--pouring the water and fertilizer in the top of the pot, or do you set the pot in water and the fertilizer until it soaks it up and shows moisture on the top. Seems to me if you are giving them fertilizer once a week it is the former.

If your pots are all right and the plants are not too wet--but still wet, then give them a full dose of fertilizer now. Mix it with water in a pan or tray that doesn't leak and set your pot in the water. Leave it for 10 or 15 minutes and then lift it out. Wait until the plant drys out. Feel the weight of a wet pot and a dry pot, by putting some promix in a pot.That's how you tell when a plant needs water.

Set it in water for a while and then add your fertilizer and put them back in and leave them there until moisture shows on top then lift them out and let the excess moisture drain off. You will find you can probably go a couple of weeks between waterings and fertilizing.

The plant has been repotted right now it's sitting in miracle potting mix. It's been in this mix for six months now.

I use peters fertilizer at 1/2 strength of 1/4 tsp per gallon.

They are in 6 inch clay pots the petunia is in a 10 inch hanging pot.

I use a fertilizer injector.

I use well water but not softened water

Well water. Have you had the water tested to determine what might be in it other than plain ol' H20? What's the pH of the water?

Last time it was checked. It was 7.8 or thereabouts. I don't
Recall the exact number right off
Hand.

I would say it's the potting mix. I use miracle grow too but I add 1/3-1/2 perlite depending on the plants water requirements. MG alone holds too much water IME. Roots also need oxygen. 6 months is a long time to be in a pot and actively growing without re-potting. I generally re-pot my tropicals every 3-4 months.
Did you notice any sewer like smell when you re-potted?

a pH of 7.8 or so is way too high. As rhizo suggested, find out about your well water.

I would check the PH of the soil. Most potting mixes contain sphagnum which has a PH of 3.0-7.5 but most are lower than 5 which will offset your higher PH water.

I'll recheck the water

We are drinking the stuff.

As for the repotting I just repotted the plant. No rotten smells. I tried different mixes. This is not the only plant that's not growing right. Different mixes and plants. Same water though.

Well water can contain lots of different minerals and even contaminants that can throw soil chemistry off. If you are drinking this water, you should have it professionally tested every year. Tested for everything, not just the pH.

qrper,

Both of those plants prefer a decent amount of Iron in their diet...more so on the Petunias. Normally lack of Iron shows up on the new growth, which looks to be the case on your Petunia photo (for containers, Ironite would be my last choice)...they are pretty heavy feeders. If your not using recommended rate of fertilizer, that could be the first part of your problem.
Now pH....7.8ish is high, but a high pH doesn't always mean you will have a strong rise in your pH. The minerals in your water (mentioned earlier) cause the pH shift...either carbonate/bicarbonate (think limestone). You can have two wells, same pH, but one has high carbonate/bicarbonate and the other has low. The one with low levels may change your pH over a long period of time, maybe not (will depend on your potting soil ingredients and sometimes fertilizer practices). But the one with high levels will, often in a short period of time. To lower your pH and supply Iron, use Iron sulfate.
If your on well water, you probably test it for safety purposes. But those tests normally wont tell you what you need to know. It may give you a pH, but not the mineral content and their levels. You need an irrigation suitability test, which will tell you what you need to know. Check any local extension office or local farmer for a lab to test irrigation water. Once you have the results, lots of information available on interpreting those results and what to do if there are problems in your water from a plants standpoint.