its fine to go into first and second although you can feel it struggle a little, but often struggles to change into 3rd and 4th. i'll put my foot down and the revs will hit the 3/4 mark, and you'll hear the motor try and change gear but just wont. youve gotta take your foot off and ease into it for it to then slowly and gradually pick up speed. the type of fuel i use makes it less noticable but it sucks not being able to go as fast and my partner haha, it wasnt always like this. i bought it off my dad a few weeks back and its like this.. it probably just needs a good service. ive checked the oil and its above the normal level.. so tis fine on that end.
my partners 04 BA MK2 changes gears with ease. 0-60 in about 4 seconds haha.
i have a feeling its probably the transmission fluid? but have a bigger feeling its something worse..

is there a way i can check it at home? im not sure if the atf is a dipstick on fords as ive only had a subaru before this.

any clues on what it could be?

Have you noticed any oil leaks at the transmission? It may be that you have insufficient oil in the Trans.
BA do not have a dipstick and you would need to take a filler plug out of the drivers side of the trans and put your little finger in, if your finger is dry, you need some oil. Make sure you get TQ95 or an equivalent.

That's a good start, after you do that and still your problem persists, you will need to get someone well versed in Autos to take a look at it.

is it in limp home mode and being electronically limited?