You might have different things going on. Your soil might be too high on acid from the long time of pine there. You might have a depletion of some nutes grass needs to thrive. You might need to fertilize. If you did a heavy fertilizer application, you might have overdone it. Depending on what your dirt is, you might need organic amending like compost. If you have a lot of root mass in the ground, that might be causing some spots to not sprout up nicely. Your grass seed might not have been effective, or snacked on by critters over the winter. If you had some odd weather recently, you might have had a sprout event that died off because of wether timing. You might need a fresh application of seed this spring.

If you can, get a basic soil test. There are kits and pH strips often sold along with other gardening supplies. That can tell you some acid/base levels and sometimes some other basics. If you want, you can send in a sample to a local service or if you have a state or county extension testing office.

A soil test is a good idea but do not use a self-test kit found at places that sell garden supplies - they are almost totally inaccurate and not worth the money they cost. Get a professional test done at a soil lab.

Plants do not make soils acidic. Soil pH is due to the underlying mineral content of the soil and the amount of rainfall. Pine and other conifer needles are acidic when in active growth but once they fall and begin to break down, they become pretty neutral. This is true of all plants that have some acidic properties. It would take truckloads of fresh material to make any sort of significant pH change. Dried needles or leaves, even accumulating for many years, will have minimal impact.

Where you are located can make a difference in a) what type of grass seed to use, and b) when to apply it. Come back with a geographical location and any photos of the area, if you can.

I suggested pH since the trees were there for over a hundred years. Probably dropped lots over time. A hundred years of drop might throw things off a bit. There's pitch/resin, and the stuff in the roots that is now going to start breaking down. Usually I wouldn't mention it- I use a lot of pine straw in my garden and I don't think it does anything for acid. But if I took down old pines, I might suspect it in the spot they were in for so long. To my understanding, pines can prefer a more acidic soil, so old trees could indicate they liked the pH of the soil. Since the OP is also saying they have crab grass that can be an indicator of higher pH. Among other things.

Anywho, you are probably right about the tests. I see them on the regular. Used to use pH strips a lot more when we were doing some experimenting. Haven't used them in years though.

As noted above, the soil's bulk density and buffering capacity limits the ability of OM/ pine leaf litter to cause a notable downward shift in soil pH. It's often assumed that fallen pine needles make the soil acidic. In reality, pines tend to grow where they find already acidic soils to their liking.

What you'll primarily be dealing with is the variable degree of allelopathic influence of volatile organic compounds (e.g., terpenes and phenolics) related to senescence of leaf litter and root exudates of pine species. Surprisingly, it's been found that during evolution of the physiological stages of pine needle decomposition from green needles < senescent needles < needle litter, the allelopathic effects of the needles on growth rate and seed germination of many herbaceous plant species is accumulative (i.e., actually increases during decomposition), which suggests that microbial populations and their activity levels have little effect insofar as limiting the allelopathic effects of pine litter and root exudates. This means it would be a very good idea to rake and dispose of fallen needles or use them where their allelopathic influence would be working for you as opposed to being at cross purposes.

Al