I am new to baking bread, and I have a recipe that calls for 2 1/4 cups total liquid to 2 3/4 cup whole wheat flour and 2 3/4 cup unbleached flour. I have made this several times, and can never mix in all of the flour, which will cause the bread to fall when baking. I do not have a mixer or bread machine, and this recipe is not for a bread machine. Help! Thank you!
This works out to a ratio of 77% hydration (see this answer for an explanation of the term), which is a very good choice for this flour mixture and should not be dry at all. So the problem is most likely in your measurement. The best you can do is to use a scale, because volume is very unreliable. In If you absolutely insist on measuring by volume, you will have to invest time in getting to do it properly - not packing, levelling well, etc.
Also, you should not be mixing the flour into the dough, you should bne kneading it. If by mixing you mean you are stirring it with a spoon, this is only an early step, you will have to continue kneading the soft dough in the bed of flour until the flour has been absorbed, and then continue for enough time to get a nice elastic dough (about 15 min of hand kneading). I am sorry if this sounds patronizing and you are already kneading properly, you did not describe it in detail so I am trying to exclude the possible culprits here.
I agree with rumtscho that the hydration (77%) is very consistent with other breads I've made, so I don't think the amounts are wrong.
Flour measurements are very prone to inaccuracy when using volume measurements. Different flours will measure differently by as much as 30% in my experience.
If you cannot use by-weight measurement, you should use proper technique in measuring flour. Spoon (with a tablespoon or similar) the flour into the measuring cup, not pressing down or packing between spoons, and when the cup is slightly over-ful take a knife or similar flat edge and scrape horizontally across the top of the measuring cup.
Also, ensure you are using a dry measuring cup, and not a liquid measuring cup; liquid measuring cups are very inaccurate for dry measurements.
Finally, be cognizant of the humidity of your environment. If you are in a very dry environment (and if you have A/C on full blast you probably are, and winter is similarly more dry than summer), you may need slightly more water, and in a very wet environment slightly less water. This is only a few tablespoons, though, it's unlikely you'd need more than that much more in a very dry environment, and in my experience most recipes are written for dry environments (as most of us cook in air-conditioned rooms) and so it's more likely you'd have too much water than too little (I usually reserve a few tablespoons from the total water if it's a new recipe and/or the humidity indoors seems high).