Userforms And Message Boxes Do Not Show Properly

I am close to finishing making up an Excel tool that takes data input from the user, does some analysis, accepts further input then prepares a report. My coding skills are not what you would call high level, so my code could probably be more efficient, but it works (well, except for this issue).

The tool opens a main multipage userform for data entry, then allows the user to view the data in the spreadsheet tables with the ability to call separate userforms to modify, delete or add records. The user then calls another userform to do some calculations and enter the appropriate results from a separate piece of software, then presses a button to prepare a report. All userforms are called from buttons in a separate userform (I'm not proficient enough and haven't had enough time to create a new ribbon in excel). At various times, messageboxes are used to convey information or prompt the user to do something - these are all simple ones with either only OK button, or OK and cancel buttons.

My problem is that when userforms are called, they sometimes appear but are totally blank (white) - see image.

blank_form.jpg

Clicking anywhere on the screen or pressing any key will bring them up properly and they seem to function normally thereafter.

In addition, the messageboxes sometimes do not appear - pressing any key will make them do so. Other than this they function normally.

The frustrating thing is that these problems occur inconsistently. I can add or change some code and it stops happening, then after testing (entering data and using the tool) starts happening again. Sometimes it just stops happening with no apparent reason, but starts again later after fixing some code or just entering some data. Not all userforms are affected at any one time and not all messageboxes have the problem at any one time.

The problem seems to occur in those subroutines where I access or manipulate data from other sheets before showing the userform or message box (but in most instances, I need to do this manipulation in order to present the correct information in the userform).

Some thought processes I have had, and unsuccessfully tried to fix the problem with (some coming from various web forums):

I thought I may have done "Application.ScreenUpdating = False" without resetting it to true, so I commented out all the "Application.ScreenUpdating = False" statements - problem stopped for a bit then started again.

Some of the userforms were modal, and even though they were not showing were still loaded so thought that this may be impacting on other userforms/message boxes, so changed all userforms to non modal - still had the problem when I opened the tool again.

Tried using "RePaint" and "DoEvents" at various points in the code after opening a userform or message box - no change.

I have tried exporting a form and its code, removing from the tool, then re-importing it. No success.

Tried the tool on another computer and the same problem occurred.