Tram the bed - The most likely reason is the bed is not level properly not the plastic arms on the CTC move to you have to check your level often (I have one as well) the Raft will cover some of this up as the first “layer” of the raft is really thick and will more likely stick to the bed.
I haven’t used a raft since figuring out the bed traming (it is a bit of an art will a bit of feeling to it).
ABS can be tricky to get to stick to a bed 110° bed temp for ABS, and it will probably need a Kapton tape surface.
PLA is a little easier, 60ish °c bed temp, and get yourself some PVA glue sticks! give the bed a wipe over with the PVA glue stick, let it dry and try that.
Printing PLA right now. Figured I would get the easy stuff down first. I’ll have to try that Glue Stick trick. heard it a few times now in various places.
Here are the pictures of the 20mm cube. If you look at the bottom, the PLA seems to be far apart in places. On the corner of one, there seems to be a separation, but it really isn’t a separation. Then on the top there seems to be a little bit of waviness going on where it looks like the plastic was pretty warm and it started to sink into the infill. The infill was only 10 % fast honeycomb.
I use a cooling fan which helps with sagging a lot. There are various designs out there - I use thing:537918 (requires a bit of extra cabling for an additional fan).
Tested did an video on youtube showing insalling some replacement arms, the ones I have bought just brace the plastic arms so just take out the bolts put the brace on and bolt up.