(long post warning. LOL so I'm sorry in advance)
Generally speaking, hot splits are for light colored garments, and cold peels are for dark colored garments. Double hot splits are also created for dark garments mainly. Double hot splits are created by screen printing the lighter colors of a design onto the transfer paper, then sending it to the dryer (just enough) and then screen printing them again to give the lighter colors enough ink to maintain opacity when split to block the colors of a dark garment.
There are a few things that can be causing some of the problems I see in your sample photos. (By the way, a lot of my practice pieces look just like yours)
First of all, we'll assume that your platens are perfectly flat, and that your heat press is reaching and maintaining the proper temperature.
If you have a Teflon pad protector installed on your lower platen, make sure that it is also perfectly flat. Feel around the edges. If you feel a lump, make sure the elastic type material that stretches over the lower platen is not still sitting (bunched up) on the top surface in some places. Pull all that elastic so it's under lower platen. Uneven pressure can cause half the transfer to stick.
Make sure your press has reached the recommended temperature for every transfer. Each time you do a transfer, the lower platen steals some heat away from the upper platen.
Before you start transferring for the day, or after you take a long enough break, pre-heat the lower platen by pressing it without a t-shirt for a minute or two. A cold lower platen can give you poor results like those in some of your pictures.
Always pre-press a new garment for about 3 to 6 seconds to get the moisture out of it. Moisture will prevent a transfer from penetrating the fabric of the garment.
Do not use hot split transfers that have light colors in them on dark garments. This will lead to the dark fabric color showing through a design color. Use Double Hot Split or Cold Peel for darks.
Next, assuming you have the right temperature, and the right transfer type, you need to play around with the time and pressure a little bit. Not enough pressure will create a poor bond between the inks and the fabric. The result might have great opacity, but a poor bond. The pressure forces the melted ink into the fabric. Too much pressure will create a great bond, but poor opacity. Not enough time, will also cause a poor bond. The inks on the transfer need to melt so they will flow into the fabric. Too much time will create poor opacity. It's all a balancing act of time, heat and pressure.
Again, assuming the proper heat was reached and maintained, I'd say that you need more pressure and possibly a little more time. Since it's easy to see if you have the temp and time set correctly, start with the pressure. You need to force that ink into the fabric. If that doesn't do it, keep the greater pressure and add a couple of seconds to the time. Next, more pressure and so on.
If you bump up the heat, reduce the time a little. If the garment color is showing through a Double hot split or Cold peel, reduce the temp a little and maybe the time by 2 seconds. Change one variable at a time and test. Change temps 5 degrees at a time and test. Change dwell times 2 seconds at a time.
I usually start at the highest end of the recommendation given by the transfer manufacturer. This way, I'm pretty sure the inks will melt and sink into the fabric. If I'm at the highest recommendations, and I'm not getting a good bond, then I crank up the pressure until that sucker sticks good.
On the other hand, if the inks soak in too much, or are not opaque enough. Back off on the pressure a little and test, then try backing off the temp or time.
Cut your practice transfers into small pieces for these tests so you don't waist them. You can also cut t-shirts into small test pieces to get the most out of them.
If it seems like I'm kind of saying the same thing over and over again with little subtle changes in my wording, that's great, because that's exactly how you dial in your heat press; little subtle changes over and over until it works.
A word (many) about cold peels. They are actually a little trickier to get right. They might look great because they are so opaque, but if the inks have not soaked into the fabric enough, they will not have a long life on the garment. If they soak in too much, they leave blotchy areas in the print. If you open the press too fast, you might hot split the edges of the design which will ruin the opacity in those areas. Plus you have to peel them at just the right time.
