Intentionally or not, you do use style sheets. Paradox will use a default if you don't specify one. It's not possible not to. In dev, do you save this form in P7 before delivering to all the customers? I'd worry the P7 instance wouldn't open a P11 form, for example, whereas P11 should open a P7 form...
It may be inconvenient for you, but the best thing you can do is to compile the form in each version and deliver the FDL to customers.
What version of Paradox is the problem customer using, and what version are you using in dev?
Back to style sheets:
WARNING: Do not change style sheets - doing so can redesign all the objects on the form to match the new style sheet. Make a backup before playing around.
In Paradox, without anything else open, go to Tools > Settings > Preferences; Forms / Reports tab. Look at the bottom. Are the "Designer style sheets:" fields populated? If not, IMO, you have an installation or configuration problem. The style sheets should be in the install directory, in a pdoxhome subdir. For example, the default install for P11 puts them here:
C:\Program Files (x86)\Corel\Paradox\Paradox\pdoxhome
DEFAULT.FP is the default for reports and DEFAULT.FT is the default for forms. If you copy these files from your dev machine, and put them into the same path (even if you must create the path) on your client machine, that may resolve the problem.
There are objectPAL methods for style sheets - getStyleSheet, setStyleSheet, saveStyleSheet. You could potentially use these in dev to create a .FT file to place on all your client computers, but they have to be in the dir where the form thinks its style sheet lives.
If none of that works, then we probably need to know a lot more about the differences between this form and other forms (including when it was created, comparatively; and what Paradox objects and features it uses that may differ from other forms; data model complexity; unique tables or indexes used) and how the client's environment (OS, Paradox version, networking, etc.) differs from dev / everyone else.
Do any other customers use this form?
FWIW,
Liz